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<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Bad block HOWTO for smartmontools</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2"><meta name="description" content="This article describes what actions might be taken when smartmontools detects a bad block on a disk. It demonstrates how to identify the file associated with an unreadable disk sector, and how to force that sector to reallocate."></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="article" title="Bad block HOWTO for smartmontools"><div class="titlepage"><div><div><h2 class="title"><a name="index"></a>Bad block HOWTO for smartmontools</h2></div><div><div class="author"><h3 class="author"><span class="firstname">Bruce</span> <span class="surname">Allen</span></h3><div class="affiliation"><div class="address"><p><br>
<code class="email">&lt;<a class="email" href="mailto:smartmontools-support@lists.sourceforge.net">smartmontools-support@lists.sourceforge.net</a>&gt;</code><br>
</p></div></div></div></div><div><div class="author"><h3 class="author"><span class="firstname">Douglas</span> <span class="surname">Gilbert</span></h3><div class="affiliation"><div class="address"><p><br>
<code class="email">&lt;<a class="email" href="mailto:smartmontools-support@lists.sourceforge.net">smartmontools-support@lists.sourceforge.net</a>&gt;</code><br>
</p></div></div></div></div><div><p class="copyright">Copyright  2004, 2005, 2006, 2007 Bruce Allen</p></div><div><div class="legalnotice" title="Legal Notice"><a name="id2541562"></a><p>
      Permission is granted to copy, distribute and/or modify this document
      under the terms of the GNU Free Documentation License, Version 1.1
      or any later version published by the Free Software Foundation;
      with no Invariant Sections, with no Front-Cover Texts, and with
      no Back-Cover Texts.
   </p><p>
    For an online copy of the license see
    <a class="ulink" href="http://www.fsf.org/copyleft/fdl.html" target="_top">
    <code class="literal">www.fsf.org/copyleft/fdl.html</code></a>.
   </p></div></div><div><p class="pubdate">2007-01-23</p></div><div><div class="revhistory"><table border="1" width="100%" summary="Revision history"><tr><th align="left" valign="top" colspan="3"><b>Revision History</b></th></tr><tr><td align="left">Revision 1.1</td><td align="left">2007-01-23</td><td align="left">dpg</td></tr><tr><td align="left" colspan="3">
             add sections on ReiserFS and partition table damage
       </td></tr><tr><td align="left">Revision 1.0</td><td align="left">2006-11-14</td><td align="left">dpg</td></tr><tr><td align="left" colspan="3">
             merge BadBlockHowTo.txt and BadBlockSCSIHowTo.txt
       </td></tr></table></div></div><div><div class="abstract" title="Abstract"><p class="title"><b>Abstract</b></p><p>
    This article describes what actions might be taken when smartmontools
    detects a bad block on a disk. It demonstrates how to identify the file
    associated with an unreadable disk sector, and how to force that sector
    to reallocate.
  </p></div></div></div><hr></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="#intro">Introduction</a></span></dt><dt><span class="sect1"><a href="#rfile">Repairs in a file system</a></span></dt><dd><dl><dt><span class="sect2"><a href="#e2_example1">ext2/ext3 first example</a></span></dt><dt><span class="sect2"><a href="#e2_example2">ext2/ext3 second example</a></span></dt><dt><span class="sect2"><a href="#unassigned">Unassigned sectors</a></span></dt><dt><span class="sect2"><a href="#reiserfs_ex">ReiserFS example</a></span></dt></dl></dd><dt><span class="sect1"><a href="#sdisk">Repairs at the disk level</a></span></dt><dd><dl><dt><span class="sect2"><a href="#partition">Partition table problems</a></span></dt><dt><span class="sect2"><a href="#lvm">LVM repairs</a></span></dt><dt><span class="sect2"><a href="#bb">Bad block reassignment</a></span></dt></dl></dd></dl></div><div class="sect1" title="Introduction"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="intro"></a>Introduction</h2></div></div></div><p>
Handling bad blocks is a difficult problem as it often involves
decisions about losing information. Modern storage devices tend
to handle the simple cases automatically, for example by writing
a disk sector that was read with difficulty to another area on
the media. Even though such a remapping can be done by a disk
drive transparently, there is still a lingering worry about media
deterioration and the disk running out of spare sectors to remap.
</p><p>
Can smartmontools help? As the <acronym class="acronym">SMART</acronym> acronym
<sup>[<a name="id2506421" href="#ftn.id2506421" class="footnote">1</a>]</sup>
suggests, the <span class="command"><strong>smartctl</strong></span> command and the
<span class="command"><strong>smartd</strong></span> daemon concentrate on monitoring and analysis.
So apart from changing some reporting settings, smartmontools will not
modify the raw data in a device. Also smartmontools only works with
physical devices, it does not know about partitions and file systems.
So other tools are needed. The job of smartmontools is to alert the user
that something is wrong and user intervention may be required.
</p><p>
When a bad block is reported one approach is to work out the mapping between
the logical block address used by a storage device and a file or some other
component of a file system using that device. Note that there may not be such
a mapping reflecting that a bad block has been found at a location not
currently used by the file system. A user may want to do this analysis to
localize and minimize the number of replacement files that are retrieved from
some backup store. This approach requires knowledge of the file system
involved and this document uses the Linux ext2/ext3 and ReiserFS file systems
for examples. Also the type of content may come into play. For example if
an area storing video has a corrupted sector, it may be easiest to accept
that a frame or two might be corrupted and instruct the disk not to retry
as that may have the visual effect of causing a momentary blank into a 1
second pause (while the disk retries the faulty sector, often accompanied
by a telltale clicking sound).
</p><p>
Another approach is to ignore the upper level consequences (e.g. corrupting
a file or worse damage to a file system) and use the facilities offered by
a storage device to repair the damage. The SCSI disk command set is used
elaborate on this low level approach.
</p></div><div class="sect1" title="Repairs in a file system"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="rfile"></a>Repairs in a file system</h2></div></div></div><p>
This section contains examples of what to do at the file system level
when smartmontools reports a bad block. These examples assume the Linux
operating system and either the ext2/ext3 or ReiserFS file system. The
various Linux commands shown have man pages and the reader is encouraged
to examine these. Of note is the <span class="command"><strong>dd</strong></span> command which is
often used in repair work
<sup>[<a name="id2506498" href="#ftn.id2506498" class="footnote">2</a>]</sup>
and has a unique command line syntax.
</p><p>
The authors would like to thank Sergey Vlasov, Theodore Ts'o,
Michael Bendzick, and others for explaining this approach. The authors would
like to add text showing how to do this for other file systems, in
particular XFS, and JFS: please email if you can provide this
information.
</p><div class="sect2" title="ext2/ext3 first example"><div class="titlepage"><div><div><h3 class="title"><a name="e2_example1"></a>ext2/ext3 first example</h3></div></div></div><p>
In this example, the disk is failing self-tests at Logical Block
Address LBA = 0x016561e9 = 23421417.  The LBA counts sectors in units
of 512 bytes, and starts at zero.
</p><p>
</p><pre class="programlisting">
root]# smartctl -l selftest /dev/hda:

SMART Self-test log structure revision number 1
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Extended offline    Completed: read failure       90%       217         0x016561e9
</pre><p>
Note that other signs that there is a bad sector on the disk can be
found in the non-zero value of the Current Pending Sector count:
</p><pre class="programlisting">
root]# smartctl -A /dev/hda
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  5 Reallocated_Sector_Ct   0x0033   100   100   005    Pre-fail  Always       -       0
196 Reallocated_Event_Count 0x0032   100   100   000    Old_age   Always       -       0
197 Current_Pending_Sector  0x0022   100   100   000    Old_age   Always       -       1
198 Offline_Uncorrectable   0x0008   100   100   000    Old_age   Offline      -       1
</pre><p>
</p><p>
First Step: We need to locate the partition on which this sector of
the disk lives:
</p><pre class="programlisting">
root]# fdisk -lu /dev/hda

Disk /dev/hda: 123.5 GB, 123522416640 bytes
255 heads, 63 sectors/track, 15017 cylinders, total 241254720 sectors
Units = sectors of 1 * 512 = 512 bytes

   Device Boot    Start       End    Blocks   Id  System
/dev/hda1   *        63   4209029   2104483+  83  Linux
/dev/hda2       4209030   5269319    530145   82  Linux swap
/dev/hda3       5269320 238227884 116479282+  83  Linux
/dev/hda4     238227885 241248104   1510110   83  Linux
</pre><p>

The partition <code class="filename">/dev/hda3</code> starts at LBA 5269320 and
extends past the 'problem' LBA.  The 'problem' LBA is offset
23421417 - 5269320 = 18152097 sectors into the partition
<code class="filename">/dev/hda3</code>.
</p><p>
To verify the type of the file system and the mount point, look in
<code class="filename">/etc/fstab</code>:
</p><pre class="programlisting">
root]# grep hda3 /etc/fstab
/dev/hda3 /data ext2 defaults 1 2
</pre><p>
You can see that this is an ext2 file system, mounted at
<code class="filename">/data</code>.
</p><p>
Second Step: we need to find the block size of the file system
(normally 4096 bytes for ext2):
</p><pre class="programlisting">
root]# tune2fs -l /dev/hda3 | grep Block
Block count:              29119820
Block size:               4096
</pre><p>
In this case the block size is 4096 bytes.

Third Step: we need to determine which File System Block contains this
LBA.  The formula is:
</p><pre class="programlisting">
  b = (int)((L-S)*512/B)
where:
b = File System block number
B = File system block size in bytes
L = LBA of bad sector
S = Starting sector of partition as shown by fdisk -lu
and (int) denotes the integer part.
</pre><p>

In our example, L=23421417, S=5269320, and B=4096.  Hence the
'problem' LBA is in block number
</p><pre class="programlisting">
   b = (int)18152097*512/4096 = (int)2269012.125
so b=2269012.
</pre><p>
</p><p>
Note: the fractional part of 0.125 indicates that this problem LBA is
actually the second of the eight sectors that make up this file system
block.
</p><p>
Fourth Step: we use debugfs to locate the inode stored in this block,
and the file that contains that inode:
</p><pre class="programlisting">
root]# debugfs
debugfs 1.32 (09-Nov-2002)
debugfs:  open /dev/hda3
debugfs:  testb 2269012
Block 2269012 not in use
</pre><p>

If the block is not in use, as in the above example, then you can skip
the rest of this step and go ahead to Step Five.
</p><p>
If, on the other hand, the block is in use, we want to identify
the file that uses it:
</p><pre class="programlisting">
debugfs:  testb 2269012
Block 2269012 marked in use
debugfs:  icheck 2269012
Block   Inode number
2269012 41032
debugfs:  ncheck 41032
Inode   Pathname
41032   /S1/R/H/714197568-714203359/H-R-714202192-16.gwf
</pre><p>
In this example, you can see that the problematic file (with the mount
point included in the path) is:
<code class="filename">/data/S1/R/H/714197568-714203359/H-R-714202192-16.gwf</code>
</p><p>
When we are working with an ext3 file system, it may happen that the
affected file is the journal itself.  Generally, if this is the case,
the inode number will be very small.  In any case, debugfs will not
be able to get the file name:
</p><pre class="programlisting">
debugfs:  testb 2269012
Block 2269012 marked in use
debugfs:  icheck 2269012
Block   Inode number
2269012 8
debugfs:  ncheck 8
Inode   Pathname
debugfs:
</pre><p>
</p><p>
To get around this situation, we can remove the journal altogether:
</p><pre class="programlisting">
tune2fs -O ^has_journal /dev/hda3
</pre><p>

and then start again with Step Four: we should see this time that the
wrong block is not in use any more.  If we removed the journal file, at
the end of the whole procedure we should remember to rebuild it:
</p><pre class="programlisting">
tune2fs -j /dev/hda3
</pre><p>
</p><p>
Fifth Step
<span class="emphasis"><em>NOTE:</em></span> This last step will <span class="emphasis"><em>permanently

</em></span> and irretrievably <span class="emphasis"><em>destroy</em></span> the contents
of the file system block that is damaged: if the block was allocated to
a file, some of the data that is in this file is going to be overwritten
with zeros.  You will not be able to recover that data unless you can
replace the file with a fresh or correct version.
</p><p>
To force the disk to reallocate this bad block we'll write zeros to
the bad block, and sync the disk:
</p><pre class="programlisting">
root]# dd if=/dev/zero of=/dev/hda3 bs=4096 count=1 seek=2269012
root]# sync
</pre><p>
</p><p>
Now everything is back to normal: the sector has been reallocated.
Compare the output just below to similar output near the top of this
article:
</p><pre class="programlisting">
root]# smartctl -A /dev/hda
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  5 Reallocated_Sector_Ct   0x0033   100   100   005    Pre-fail  Always       -       1
196 Reallocated_Event_Count 0x0032   100   100   000    Old_age   Always       -       1
197 Current_Pending_Sector  0x0022   100   100   000    Old_age   Always       -       0
198 Offline_Uncorrectable   0x0008   100   100   000    Old_age   Offline      -       1
</pre><p>

Note: for some disks it may be necessary to update the SMART Attribute values by using
<span class="command"><strong>smartctl -t offline /dev/hda</strong></span>
</p><p>
We have corrected the first errored block.  If more than one blocks
were errored, we should repeat all the steps for the subsequent ones.
After we do that, the disk will pass its self-tests again:

</p><pre class="programlisting">
root]# smartctl -t long /dev/hda  [wait until test completes, then]
root]# smartctl -l selftest /dev/hda

SMART Self-test log structure revision number 1
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Extended offline    Completed without error       00%       239         -
# 2  Extended offline    Completed: read failure       90%       217         0x016561e9
# 3  Extended offline    Completed: read failure       90%       212         0x016561e9
# 4  Extended offline    Completed: read failure       90%       181         0x016561e9
# 5  Extended offline    Completed without error       00%        14         -
# 6  Extended offline    Completed without error       00%         4         -
</pre><p>
</p><p>
and no longer shows any offline uncorrectable sectors:

</p><pre class="programlisting">
root]# smartctl -A /dev/hda
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  5 Reallocated_Sector_Ct   0x0033   100   100   005    Pre-fail  Always       -       1
196 Reallocated_Event_Count 0x0032   100   100   000    Old_age   Always       -       1
197 Current_Pending_Sector  0x0022   100   100   000    Old_age   Always       -       0
198 Offline_Uncorrectable   0x0008   100   100   000    Old_age   Offline      -       0
</pre><p>
</p></div><div class="sect2" title="ext2/ext3 second example"><div class="titlepage"><div><div><h3 class="title"><a name="e2_example2"></a>ext2/ext3 second example</h3></div></div></div><p>
On this drive, the first sign of trouble was this email from smartd:
</p><pre class="programlisting">
    To: ballen
    Subject: SMART error (selftest) detected on host: medusa-slave166.medusa.phys.uwm.edu

    This email was generated by the smartd daemon running on host:
    medusa-slave166.medusa.phys.uwm.edu in the domain: master001-nis

    The following warning/error was logged by the smartd daemon:
    Device: /dev/hda, Self-Test Log error count increased from 0 to 1
</pre><p>
</p><p>
Running <span class="command"><strong>smartctl -a /dev/hda</strong></span> confirmed the problem:
    
</p><pre class="programlisting">
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Extended offline    Completed: read failure       80%       682         0x021d9f44

Note that the failing LBA reported is 0x021d9f44 (base 16) = 35495748 (base 10)
    
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  5 Reallocated_Sector_Ct   0x0033   100   100   005    Pre-fail  Always       -       0
196 Reallocated_Event_Count 0x0032   100   100   000    Old_age   Always       -       0
197 Current_Pending_Sector  0x0022   100   100   000    Old_age   Always       -       3
198 Offline_Uncorrectable   0x0008   100   100   000    Old_age   Offline      -       3
</pre><p>
</p><p>
and one can see above that there are 3 sectors on the list of pending
sectors that the disk can't read but would like to reallocate.
</p><p>
The device also shows errors in the SMART error log:
</p><pre class="programlisting">
Error 212 occurred at disk power-on lifetime: 690 hours
  After command completion occurred, registers were:
  ER ST SC SN CL CH DH
  -- -- -- -- -- -- --
  40 51 12 46 9f 1d e2  Error: UNC 18 sectors at LBA = 0x021d9f46 = 35495750

  Commands leading to the command that caused the error were:
  CR FR SC SN CL CH DH DC   Timestamp  Command/Feature_Name
  -- -- -- -- -- -- -- --   ---------  --------------------
  25 00 12 46 9f 1d e0 00 2485545.000  READ DMA EXT
</pre><p>
</p><p>
Signs of trouble at this LBA may also be found in SYSLOG:
</p><pre class="programlisting">
[root]# grep LBA /var/log/messages | awk '{print $12}' | sort | uniq
 LBAsect=35495748
 LBAsect=35495750
</pre><p>
</p><p>
So I decide to do a quick check to see how many bad sectors there
really are. Using the bash shell I check 70 sectors around the trouble
area:
</p><pre class="programlisting">
[root]# export i=35495730
[root]# while [ $i -lt 35495800 ]
        &gt; do echo $i
        &gt; dd if=/dev/hda of=/dev/null bs=512 count=1 skip=$i
        &gt; let i+=1
        &gt; done
 
&lt;SNIP&gt;   

35495734
1+0 records in
1+0 records out
35495735
dd: reading `/dev/hda': Input/output error
0+0 records in
0+0 records out

&lt;SNIP&gt;

35495751
dd: reading `/dev/hda': Input/output error
0+0 records in
0+0 records out
35495752
1+0 records in
1+0 records out

&lt;SNIP&gt;
</pre><p>
</p><p>
which shows that the seventeen sectors 35495735-35495751 (inclusive)
are not readable.
</p><p>
Next, we identify the files at those locations.  The partitioning
information on this disk is identical to the first example above, and
as in that case the problem sectors are on the third partition
<code class="filename">/dev/hda3</code>.  So we have:
</p><pre class="programlisting">
     L=35495735 to 35495751
     S=5269320
     B=4096
</pre><p>
so that b=3778301 to 3778303 are the three bad blocks in the file
system.

</p><pre class="programlisting">
[root]# debugfs
debugfs 1.32 (09-Nov-2002)
debugfs:  open /dev/hda3
debugfs:  icheck 3778301
Block   Inode number
3778301 45192
debugfs:  icheck 3778302
Block   Inode number
3778302 45192
debugfs:  icheck 3778303
Block   Inode number
3778303 45192
debugfs:  ncheck 45192
Inode   Pathname
45192   /S1/R/H/714979488-714985279/H-R-714979984-16.gwf
debugfs:  quit
</pre><p>
Note that the first few steps of this procedure could also be done
with a single command, which is very helpful if there are many bad
blocks (thanks to Danie Marais for pointing this out):
</p><pre class="programlisting">
debugfs: icheck 3778301 3778302 3778303
</pre><p>
</p><p>
And finally, just to confirm that this is really the damaged file:
</p><p>
</p><pre class="programlisting">
[root]# md5sum /data/S1/R/H/714979488-714985279/H-R-714979984-16.gwf
md5sum: /data/S1/R/H/714979488-714985279/H-R-714979984-16.gwf: Input/output error
</pre><p>
</p><p>
Finally we force the disk to reallocate the three bad blocks:
</p><pre class="programlisting">
[root]# dd if=/dev/zero of=/dev/hda3 bs=4096 count=3 seek=3778301
[root]# sync
</pre><p>
</p><p>
We could also probably use:
</p><pre class="programlisting">
[root]# dd if=/dev/zero of=/dev/hda bs=512 count=17 seek=35495735
</pre><p>
</p><p>
At this point we now have:
</p><pre class="programlisting">
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  5 Reallocated_Sector_Ct   0x0033   100   100   005    Pre-fail  Always       -       0
196 Reallocated_Event_Count 0x0032   100   100   000    Old_age   Always       -       0
197 Current_Pending_Sector  0x0022   100   100   000    Old_age   Always       -       0
198 Offline_Uncorrectable   0x0008   100   100   000    Old_age   Offline      -       0
</pre><p>
</p><p>
which is encouraging, since the pending sectors count is now zero.
Note that the drive reallocation count has not yet increased: the
drive may now have confidence in these sectors and have decided not to
reallocate them..
</p><p>
A device self test: 
</p><pre class="programlisting">
  [root#] smartctl -t long /dev/hda
(then wait about an hour) shows no unreadable sectors or errors:

Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Extended offline    Completed without error       00%       692         -
# 2  Extended offline    Completed: read failure       80%       682         0x021d9f44
</pre><p>
</p></div><div class="sect2" title="Unassigned sectors"><div class="titlepage"><div><div><h3 class="title"><a name="unassigned"></a>Unassigned sectors</h3></div></div></div><p>
This section was written by Kay Diederichs. Even though this section
assumes Linux and the ext2/ext3 file system, the strategy should be
more generally applicable.
</p><p>
I read your badblocks-howto at and greatly
benefited from it. One thing that's (maybe) missing is that often the
<span class="command"><strong>smartctl -t long</strong></span> scan finds a bad sector which is
<span class="emphasis"><em> not</em></span> assigned to
any file. In that case it does not help to run debugfs, or rather
debugfs reports the fact that no file owns that sector. Furthermore,
it is somewhat laborious to come up with the correct numbers for
debugfs, and debugfs is slow ...
</p><p>
So what I suggest in the case of presence of
Current_Pending_Sector/Offline_Uncorrectable errors is to create a
huge file on that file system.
</p><pre class="programlisting">
  dd if=/dev/zero of=/some/mount/point bs=4k
</pre><p>
creates the file. Leave it running until the partition/file system is
full. This will make the disk reallocate those sectors which do not
belong to a file. Check the <span class="command"><strong>smartctl -a</strong></span> output after
that and make
sure that the sectors are reallocated. If any remain, use the debugfs
method.  Of course the usual caveats apply - back it up first, and so
on.
</p></div><div class="sect2" title="ReiserFS example"><div class="titlepage"><div><div><h3 class="title"><a name="reiserfs_ex"></a>ReiserFS example</h3></div></div></div><p>
This section was written by Joachim Jautz with additions from Manfred
Schwarb.
</p><p>
The following problems were reported during a scheduled test:
</p><pre class="programlisting">
smartd[575]: Device: /dev/hda, starting scheduled Offline Immediate Test.
[... 1 hour later ...]
smartd[575]: Device: /dev/hda, 1 Currently unreadable (pending) sectors
smartd[575]: Device: /dev/hda, 1 Offline uncorrectable sectors
</pre><p>
</p><p>
[Step 0] The SMART selftest/error log
(see <span class="command"><strong>smartctl -l selftest</strong></span>) indicated there was a problem
with block address (i.e. the 512 byte sector at) 58656333. The partition
table (e.g. see <span class="command"><strong>sfdisk -luS /dev/hda</strong></span> or
<span class="command"><strong>fdisk -ul /dev/hda</strong></span>) indicated that this block was in the
<code class="filename">/dev/hda3</code> partition which contained a ReiserFS file
system. That partition started at block address 54781650.
</p><p>
While doing the initial analysis it may also be useful to take a copy
of the disk attributes returned by <span class="command"><strong>smartctl -A /dev/hda</strong></span>.
Specifically the values associated with the "Reallocated_Sector_Ct" and
"Reallocated_Event_Count" attributes (for ATA disks, the grown list (GLIST)
length for SCSI disks). If these are incremented at the end of the procedure
it indicates that the disk has re-allocated one or more sectors.
</p><p>
[Step 1] Get the file system's block size:
</p><pre class="programlisting">
# debugreiserfs /dev/hda3 | grep '^Blocksize'
Blocksize: 4096
</pre><p>
</p><p>
[Step 2] Calculate the block number:
</p><pre class="programlisting">
# echo "(58656333-54781650)*512/4096" | bc -l
484335.37500000000000000000
</pre><p>
It is re-assuring that the calculated 4 KB damaged block address in
<code class="filename">/dev/hda3</code> is less than "Count of blocks on the
device" shown in the output of <span class="command"><strong>debugreiserfs</strong></span> shown above.
</p><p>
[Step 3] Try to get more info about this block =&gt; reading the block
fails as expected but at least we see now that it seems to be unused.
If we do not get the `Cannot read the block' error we should
check if our calculation in [Step 2] was correct ;)
</p><pre class="programlisting">
# debugreiserfs -1 484335 /dev/hda3
debugreiserfs 3.6.19 (2003 http://www.namesys.com)

484335 is free in ondisk bitmap
The problem has occurred looks like a hardware problem.
</pre><p>
</p><p>
If you have bad blocks, we advise you to get a new hard drive, because
once you get one bad block that the disk drive internals cannot hide from
your sight, the chances of getting more are generally said to become
much higher (precise statistics are unknown to us), and this disk
drive is probably not expensive enough for you to risk your
time and data on it. If you don't want to follow that 
advice then if you have just a few bad blocks, try writing to the
bad blocks and see if the drive remaps the bad blocks (that means
it takes a block it has in reserve and allocates it for use for
of that block number). If it cannot remap the block, use
<span class="command"><strong>badblock</strong></span> option (-B) with reiserfs utils to handle
this block correctly.
</p><pre class="programlisting">
bread: Cannot read the block (484335): (Input/output error).

Aborted
</pre><p>
So it looks like we have the right (i.e. faulty) block address.
</p><p>
[Step 4] Try then to find the affected file
<sup>[<a name="id2550815" href="#ftn.id2550815" class="footnote">3</a>]</sup>:
</p><pre class="programlisting">
tar -cO /mydir | cat &gt;/dev/null
</pre><p>
If you do not find any unreadable files, then the block may be free or
located in some metadata of the file system.
</p><p>
[Step 5] Try your luck: bang the affected block with
<span class="command"><strong>badblocks -n</strong></span> (non-destructive read-write mode, do unmount
first), if you are very lucky the failure is transient and you can provoke
reallocation
<sup>[<a name="id2550862" href="#ftn.id2550862" class="footnote">4</a>]</sup>:
</p><pre class="programlisting">
# badblocks -b 4096 -p 3 -s -v -n /dev/hda3 `expr 484335 + 100` `expr 484335 - 100`
</pre><p>
<sup>[<a name="id2550876" href="#ftn.id2550876" class="footnote">5</a>]</sup>
</p><p>
check success with <span class="command"><strong>debugreiserfs -1 484335 /dev/hda3</strong></span>.
Otherwise:
</p><p>
[Step 6] Perform this step <span class="emphasis"><em>only</em></span> if Step 5 has failed
to fix the problem: overwrite that block to force reallocation:
</p><pre class="programlisting">
# dd if=/dev/zero of=/dev/hda3 count=1 bs=4096 seek=484335
1+0 records in
1+0 records out
4096 bytes transferred in 0.007770 seconds (527153 bytes/sec)
</pre><p>
</p><p>
[Step 7] If you can't rule out the bad block being in metadata, do
a file system check:
</p><pre class="programlisting">
reiserfsck --check
</pre><p>
This could take a long time so you probably better go for lunch ...
</p><p>
[Step 8] Proceed as stated earlier. For example, sync disk and run a long
selftest that should succeed now.
</p></div></div><div class="sect1" title="Repairs at the disk level"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="sdisk"></a>Repairs at the disk level</h2></div></div></div><p>
This section first looks at a damaged partition table. Then it ignores
the upper level impact of a bad block and just repairs the underlying
sector so that defective sector will not cause problems in the future.
</p><div class="sect2" title="Partition table problems"><div class="titlepage"><div><div><h3 class="title"><a name="partition"></a>Partition table problems</h3></div></div></div><p>
Some software failures can lead to zeroes or random data being written
on the first block of a disk. For disks that use a DOS-based partitioning
scheme this will overwrite the partition table which is found at the
end of the first block. This is a single point of failure so after the
damage tools like <span class="command"><strong>fdisk</strong></span> have no alternate data to use
so they report no partitions or a damaged partition table.
</p><p>
One utility that may help is 
<a class="ulink" href="http://www.cgsecurity.org/wiki/TestDisk" target="_top">
<code class="literal">testdisk</code></a> which can scan a disk looking for
partitions and recreate a partition table if requested.
<sup>[<a name="id2550980" href="#ftn.id2550980" class="footnote">6</a>]</sup>
</p><p>
Programs that create DOS partitions
often place the first partition at logical block address 63. In Linux
a loop back mount can be attempted at the appropriate offset of a disk
with a damaged partition table. This approach may involve placing the
disk with the damaged partition table in a working computer or perhaps
an external USB enclosure. Assuming the disk with the damaged partition
is <code class="filename">/dev/hdb</code>. Then the following read-only loop back
mount could be tried:
</p><pre class="programlisting">
# mount -r /dev/hdb -o loop,offset=32256 /mnt
</pre><p>
The offset is in bytes so the number given is (63 * 512). If the file
system cannot be identified then a '-t &lt;fs_type&gt;'
may be needed (although this is not a good sign). If this mount is
successful, a backup procedure is advised.
</p><p>
Only the primary DOS partitions are recorded in the first block of
a disk. The extended DOS partition table is placed elsewhere on
a disk. Again there is only one copy of it so it represents another
single point of failure. All DOS partition information can be
read in a form that can be used to recreate the tables with the
<span class="command"><strong>sfdisk</strong></span> command. Obviously this needs to be done
beforehand and the file put on other media. Here is how to fetch the
partition table information:
</p><pre class="programlisting">
# sfdisk -dx /dev/hda &gt; my_disk_partition_info.txt
</pre><p>
Then <code class="filename">my_disk_partition_info.txt</code> should be placed on
other media. If disaster strikes, then the disk with the damaged partition
table(s) can be placed in a working system, let us say the damaged disk is
now at <code class="filename">/dev/hdc</code>, and the following command restores
the partition table(s):
</p><pre class="programlisting">
# sfdisk -x -O part_block_prior.img /dev/hdc &lt; my_disk_partition_info.txt
</pre><p>
Since the above command is potentially destructive it takes a copy of the
block(s) holding the partition table(s) and puts it in
<code class="filename">part_block_prior.img</code> prior to any changes. Then it
changes the partition tables as indicated by
<code class="filename">my_disk_partition_info.txt</code>. For what it is worth the
author did test this on his system!
<sup>[<a name="id2551099" href="#ftn.id2551099" class="footnote">7</a>]</sup>
</p><p>
For creating, destroying, resizing, checking and copying partitions, and
the file systems on them, GNU's
<a class="ulink" href="http://www.gnu.org/software/parted" target="_top">
<code class="literal">parted</code></a> is worth examining.
The <a class="ulink" href="http://www.tldp.org/HOWTO/Large-Disk-HOWTO.html" target="_top">
<code class="literal">Large Disk HOWTO</code></a> is also a useful resource.
</p></div><div class="sect2" title="LVM repairs"><div class="titlepage"><div><div><h3 class="title"><a name="lvm"></a>LVM repairs</h3></div></div></div><p>
This section was written by Frederic BOITEUX. It was titled: "HOW TO
LOCATE AND REPAIR BAD BLOCKS ON AN LVM VOLUME".
</p><p>
Smartd reports an error in a short test:
</p><pre class="programlisting">
# smartctl -a /dev/hdb
...
SMART Self-test log structure revision number 1
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Short offline       Completed: read failure       90%        66         37383668
</pre><p>
So the disk has a bad block located in LBA block 37383668
</p><p>
In which physical partition is the bad block?
</p><pre class="programlisting">
# sfdisk -luS /dev/hdb  # or 'fdisk -ul /dev/hdb'

Disk /dev/hdb: 9729 cylinders, 255 heads, 63 sectors/track
Units = sectors of 512 bytes, counting from 0

   Device Boot    Start       End   #sectors  Id  System
/dev/hdb1            63    996029     995967  82  Linux swap / Solaris
/dev/hdb2   *    996030   1188809     192780  83  Linux
/dev/hdb3       1188810 156296384  155107575  8e  Linux LVM
/dev/hdb4             0         -          0   0  Empty
</pre><p>

It's in the <code class="filename">/dev/hdb3</code> partition, a LVM2 partition.
From the LVM2 partition beginning, the bad block has an offset of
</p><pre class="programlisting">
(37383668 - 1188810) = 36194858
</pre><p>
</p><p>
We have to find in which LVM2 logical partition the block belongs to.
</p><p>
In which logical partition is the bad block?
</p><p>
<span class="emphasis"><em>IMPORTANT</em></span>: LVM2 can use different schemes dividing
its physical partitions to logical ones: linear, striped, contiguous or
 not... The following example assumes that allocation is linear!
</p><p>
The physical partition used by LVM2 is divided in PE (Physical Extent)
units of the same size, starting at pe_start' 512 bytes blocks from
the beginning of the physical partition.
</p><p>
The 'pvdisplay' command gives the size of the PE (in KB) of the
LVM partition:
</p><pre class="programlisting">
#  part=/dev/hdb3; pvdisplay -c $part | awk -F: '{print $8}'
4096
</pre><p>
</p><p>
To get its size in LBA block size (512 bytes or 0.5 KB), we multiply this
number by 2: 4096 * 2 = 8192 blocks for each PE.
</p><p>
To find the offset from the beginning of the physical partition is a
bit more difficult: if you have a recent LVM2 version, try:
</p><pre class="programlisting">
# pvs -o+pe_start $part
</pre><p>
</p><p>
Either, you can look in /etc/lvm/backup:
</p><pre class="programlisting">
# grep pe_start $(grep -l $part /etc/lvm/backup/*)
                        pe_start = 384
</pre><p>
</p><p>
Then, we search in which PE is the badblock, calculating the PE rank
in which the faulty block of the partition is:
physical partition's bad block number / sizeof(PE) =
</p><pre class="programlisting">
36194858 / 8192 = 4418.3176
</pre><p>
</p><p>
So we have to find in which LVM2 logical partition is used the PE
number 4418 (count starts from 0):
</p><pre class="programlisting">
# lvdisplay --maps |egrep 'Physical|LV Name|Type'
  LV Name                /dev/WDC80Go/racine
    Type                linear
    Physical volume     /dev/hdb3
    Physical extents    0 to 127
  LV Name                /dev/WDC80Go/usr
    Type                linear
    Physical volume     /dev/hdb3
    Physical extents    128 to 1407
  LV Name                /dev/WDC80Go/var
    Type                linear
    Physical volume     /dev/hdb3
    Physical extents    1408 to 1663
  LV Name                /dev/WDC80Go/tmp
    Type                linear
    Physical volume     /dev/hdb3
    Physical extents    1664 to 1791
  LV Name                /dev/WDC80Go/home
    Type                linear
    Physical volume     /dev/hdb3
    Physical extents    1792 to 3071
  LV Name                /dev/WDC80Go/ext1
    Type                linear
    Physical volume     /dev/hdb3
    Physical extents    3072 to 10751
  LV Name                /dev/WDC80Go/ext2
    Type                linear
    Physical volume     /dev/hdb3
    Physical extents    10752 to 18932
</pre><p>
</p><p>
So the PE #4418 is in the <code class="filename">/dev/WDC80Go/ext1</code>
LVM logical partition.
</p><p>
Size of logical block of file system on <code class="filename">/dev/WDC80Go/ext1
</code>:
</p><p>
It's a ext3 fs, so I get it like this:
</p><pre class="programlisting">
# dumpe2fs /dev/WDC80Go/ext1 | grep 'Block size'
dumpe2fs 1.37 (21-Mar-2005)
Block size:               4096
</pre><p>
</p><p>
bad block number for the file system:
</p><p>
The logical partition begins on PE 3072:
</p><pre class="programlisting">
 (# PE's start of partition * sizeof(PE)) + parttion offset[pe_start] =
 (3072 * 8192) + 384 = 25166208
</pre><p>
512b block of the physical partition, so the bad block number for the
file system is:
</p><pre class="programlisting">
(36194858 - 25166208) / (sizeof(fs block) / 512)
= 11028650 / (4096 / 512)  = 1378581.25
</pre><p>
</p><p>
Test of the fs bad block:
</p><pre class="programlisting">
dd if=/dev/WDC80Go/ext1 of=block1378581 bs=4096 count=1 skip=1378581
</pre><p>
</p><p>
If this dd command succeeds, without any error message in console or
syslog, then the block number calculation is probably wrong! *Don't*
go further, re-check it and if you don't find the error, please
renounce!
</p><p>
Search / correction follows the same scheme as for simple
partitions:
</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p>
find possible impacted files with debugfs (icheck &lt;fs block nb&gt;,
then ncheck &lt;icheck nb&gt;).
</p></li><li class="listitem"><p>
reallocate bad block writing zeros in it, *using the fs block size*:
</p></li></ul></div><p>
</p><p>
</p><pre class="programlisting">
dd if=/dev/zero of=/dev/WDC80Go/ext1 count=1 bs=4096 seek=1378581
</pre><p>
</p><p>
Et voil!
</p></div><div class="sect2" title="Bad block reassignment"><div class="titlepage"><div><div><h3 class="title"><a name="bb"></a>Bad block reassignment</h3></div></div></div><p>
The SCSI disk command set and associated disk architecture are assumed
in this section. SCSI disks have their own logical to physical mapping
allowing a damaged sector (usually carrying 512 bytes of data) to be
remapped irrespective of the operating system, file system or software
RAID being used.
</p><p>
The terms <span class="emphasis"><em>block</em></span> and <span class="emphasis"><em>sector</em></span> are
used interchangeably, although block tends to get used in higher level or
more abstract contexts such as a <span class="emphasis"><em>logical block</em></span>.
</p><p>
When a SCSI disk is formatted, defective sectors identified during
the manufacturing process (the so called primary list: PLIST),
those found during the format itself (the certification list: CLIST),
those given explicitly to the format command (the DLIST) and optionally
the previous grown list (GLIST) are not used in the logical block
map. The number (and low level addresses) of the unmapped sectors can be
found with the READ DEFECT DATA SCSI command.
</p><p>
SCSI disks tend to be divided into zones which have spare sectors and
perhaps spare tracks, to support the logical block address mapping
process. The idea is that if a logical block is remapped, the heads do not
have to move a long way to access the replacement sector. Note that spare
sectors are a scarce resource.
</p><p>
Once a SCSI disk format has completed successfully, other problems
may appear over time. These fall into two categories:
</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p>
recoverable: the Error Correction Codes (ECC) detect a problem
but it is small enough to be corrected. Optionally other strategies
such as retrying the access may retrieve the data.
</p></li><li class="listitem"><p>
unrecoverable: try as it may, the disk logic and ECC algorithms
cannot recover the data. This is often reported as a
<span class="emphasis"><em>medium error</em></span>.
</p></li></ul></div><p>
</p><p>
Other things can go wrong, typically associated with the transport and
they will be reported using a term other than
<span class="emphasis"><em>medium error</em></span>. For example a disk may decide a read
operation was successful but a computer's host bus adapter (HBA) checking
the incoming data detects a CRC error due to a bad cable or termination.
</p><p>
Depending on the disk vendor, recoverable errors can be ignored. After all,
some disks have up to 68 bytes of ECC above the payload size of 512 bytes
so why use up spare sectors which are limited in number
<sup>[<a name="id2551516" href="#ftn.id2551516" class="footnote">8</a>]</sup>
?
If the disk can recover the data and does decide to re-allocate (reassign)
a sector, then first it checks the settings of the ARRE and AWRE bits in the
read-write error recovery mode page. Usually these bits are set
<sup>[<a name="id2551535" href="#ftn.id2551535" class="footnote">9</a>]</sup>
enabling automatic (read or write) re-allocation. The automatic
re-allocation may also fail if the zone (or disk) has run out of spare
sectors.
</p><p>
Another consideration with RAIDs, and applications that require a high
data rate without pauses, is that the controller logic may not want a
disk to spend too long trying to recover an error.
</p><p>
Unrecoverable errors will cause a <span class="emphasis"><em>medium error</em></span> sense
key, perhaps with some useful additional sense information. If the extended
background self test includes a full disk read scan, one would expect the
self test log to list the bad block, as shown in the <a class="xref" href="#rfile" title="Repairs in a file system">the section called &#8220;Repairs in a file system&#8221;</a>.
Recent SCSI disks with a periodic background scan should also list
unrecoverable read errors (and some recoverable errors as well). The
advantage of the background scan is that it runs to completion while self
tests will often terminate at the first serious error.
</p><p>
SCSI disks expect unrecoverable errors to be fixed manually using the
REASSIGN BLOCKS SCSI command since loss of data is involved. It is possible
that an operating system or a file system could issue the REASSIGN BLOCKS
command itself but the authors are unaware of any examples. The REASSIGN BLOCKS
command will reassign one or more blocks, attempting to (partially ?) recover
the data (a forlorn hope at this stage), fetch an unused spare sector from the
current zone while adding the damaged old sector to the GLIST (hence the
name "grown" list). The contents of the GLIST may not be that interesting
but <span class="command"><strong>smartctl</strong></span> prints out the number of entries in the grown
list and if that number grows quickly, the disk may be approaching the end
of its useful life.
</p><p>
Here is an alternate brute force technique to consider: if the data on the
SCSI or ATA disk has all been backed up (e.g. is held on the other disks in
a RAID 5 enclosure), then simply reformatting the disk may be the least
cumbersome approach.
</p><div class="sect3" title="Example"><div class="titlepage"><div><div><h4 class="title"><a name="sexample"></a>Example</h4></div></div></div><p>
Given a "bad block", it still may be useful to look at the
<span class="command"><strong>fdisk</strong></span> command (if the disk has multiple partitions)
to find out which partition is involved, then use
<span class="command"><strong>debugfs</strong></span> (or a similar tool for the file system in
question) to find out which, if any, file or other part of the file system
may have been damaged. This is discussed in the <a class="xref" href="#rfile" title="Repairs in a file system">the section called &#8220;Repairs in a file system&#8221;</a>.
</p><p>
Then a program that can execute the REASSIGN BLOCKS SCSI command is
required. In Linux (2.4 and 2.6 series), FreeBSD, Tru64(OSF) and Windows
the author's <span class="command"><strong>sg_reassign</strong></span> utility in the sg3_utils
package can be used. Also found in that package is
<span class="command"><strong>sg_verify</strong></span> which can be used to check that a block is
readable.
</p><p>
Assume that logical block address 1193046 (which is 123456 in hex) is
corrupt
<sup>[<a name="id2551756" href="#ftn.id2551756" class="footnote">10</a>]</sup>
on the disk at <code class="filename">/dev/sdb</code>. A long selftest command like
<span class="command"><strong>smartctl -t long /dev/sdb</strong></span> may result in log results
like this:
</p><pre class="programlisting">
# smartctl -l selftest /dev/sdb
smartctl version 5.37 [i686-pc-linux-gnu] Copyright (C) 2002-6 Bruce Allen
Home page is http://smartmontools.sourceforge.net/


SMART Self-test log
Num  Test              Status            segment  LifeTime  LBA_first_err [SK ASC ASQ]
     Description                         number   (hours)
# 1  Background long   Failed in segment      -     354           1193046 [0x3 0x11 0x0]
# 2  Background short  Completed              -     323                 - [-   -    -]
# 3  Background short  Completed              -     194                 - [-   -    -]
</pre><p>
</p><p>
The <span class="command"><strong>sg_verify</strong></span> utility can be used to confirm that there
is a problem at that address:
</p><pre class="programlisting">
# sg_verify --lba=1193046 /dev/sdb
verify (10):  Fixed format, current;  Sense key: Medium Error
 Additional sense: Unrecovered read error
  Info fld=0x123456 [1193046]
  Field replaceable unit code: 228
  Actual retry count: 0x008b
medium or hardware error, reported lba=0x123456
</pre><p>
</p><p>
Now the GLIST length is checked before the block reassignment:
</p><pre class="programlisting">
# sg_reassign --grown /dev/sdb
&gt;&gt; Elements in grown defect list: 0
</pre><p>
</p><p>
And now for the actual reassignment followed by another check of the GLIST
length:
</p><pre class="programlisting">
# sg_reassign --address=1193046 /dev/sdb

# sg_reassign --grown /dev/sdb
&gt;&gt; Elements in grown defect list: 1
</pre><p>
</p><p>
The GLIST length has grown by one as expected. If the disk was unable to
recover any data, then the "new" block at lba 0x123456 has vendor specific
data in it. The <span class="command"><strong>sg_reassign</strong></span> utility can also do bulk
reassigns, see <span class="command"><strong>man sg_reassign</strong></span> for more information.
</p><p>
The <span class="command"><strong>dd</strong></span> command could be used to read the contents of
the "new" block:
</p><pre class="programlisting">
# dd if=/dev/sdb iflag=direct skip=1193046 of=blk.img bs=512 count=1
</pre><p>
</p><p>
and a hex editor
<sup>[<a name="id2551874" href="#ftn.id2551874" class="footnote">11</a>]</sup>
used to view and potentially change the
<code class="filename">blk.img</code> file. An altered <code class="filename">blk.img</code>
file (or <code class="filename">/dev/zero</code>) could be written back with:
</p><pre class="programlisting">
# dd if=blk.img of=/dev/sdb seek=1193046 oflag=direct bs=512 count=1
</pre><p>
</p><p>
More work may be needed at the file system level, especially if the
reassigned block held critical file system information such as
a superblock or a directory.
</p><p>
Even if a full backup of the disk is available, or the disk has been
"ejected" from a RAID, it may still be worthwhile to reassign the bad
block(s) that caused the problem (or simply format the disk (see
<span class="command"><strong>sg_format</strong></span> in the sg3_utils package)) and re-use the
disk later (not unlike the way a replacement disk from a manufacturer
might be used).
</p><p>
$Id: badblockhowto.xml 2873 2009-08-11 21:46:20Z dipohl $
</p></div></div></div><div class="footnotes"><br><hr width="100" align="left"><div class="footnote"><p><sup>[<a name="ftn.id2506421" href="#id2506421" class="para">1</a>] </sup>
Self-Monitoring, Analysis and Reporting Technology -&gt; SMART
</p></div><div class="footnote"><p><sup>[<a name="ftn.id2506498" href="#id2506498" class="para">2</a>] </sup>
Starting with GNU coreutils release 5.3.0, the <span class="command"><strong>dd</strong></span>
command in Linux includes the options 'iflag=direct' and 'oflag=direct'.
Using these with the <span class="command"><strong>dd</strong></span> commands should be helpful,
because adding these flags should avoid any interaction
with the block buffering IO layer in Linux and permit direct reads/writes
from the raw device.  Use <span class="command"><strong>dd --help</strong></span> to see if your
version of dd supports these options. If not, the latest code for dd
can be found at <a class="ulink" href="http://alpha.gnu.org/gnu/coreutils" target="_top">
<code class="literal">alpha.gnu.org/gnu/coreutils</code></a>.
</p></div><div class="footnote"><p><sup>[<a name="ftn.id2550815" href="#id2550815" class="para">3</a>] </sup>
Do not use <span class="command"><strong>tar -c -f /dev/null</strong></span> or
<span class="command"><strong>tar -cO /mydir &gt;/dev/null</strong></span>. GNU tar does not
actually read the files if <code class="filename">/dev/null</code> is used as
archive path or as standard output, see <span class="command"><strong>info tar</strong></span>.
</p></div><div class="footnote"><p><sup>[<a name="ftn.id2550862" href="#id2550862" class="para">4</a>] </sup>
Important: set blocksize range is arbitrary, but do not only test a single
block, as bad blocks are often social. Not too large as this test probably
has not 0% risk.
</p></div><div class="footnote"><p><sup>[<a name="ftn.id2550876" href="#id2550876" class="para">5</a>] </sup>
The rather awkward `expr 484335 + 100` (note the back quotes) can be replaced
with $((484335+100)) if the bash shell is being used. Similarly the last
argument can become $((484335-100)) .
</p></div><div class="footnote"><p><sup>[<a name="ftn.id2550980" href="#id2550980" class="para">6</a>] </sup>
<span class="command"><strong>testdisk</strong></span> scans the media for the beginning of file
systems that it recognizes. It can be tricked by data that looks
like the beginning of a file system or an old file system from a
previous partitioning of the media (disk). So care should be taken.
Note that file systems should not overlap apart from the fact that
extended partitions lie wholly within a extended partition table
allocation. Also if the root partition of a Linux/Unix installation
can be found then the <code class="filename">/etc/fstab</code> file is a useful
resource for finding the partition numbers of other partitions.
</p></div><div class="footnote"><p><sup>[<a name="ftn.id2551099" href="#id2551099" class="para">7</a>] </sup>
Thanks to Manfred Schwarb for the information about storing partition
table(s) beforehand.
</p></div><div class="footnote"><p><sup>[<a name="ftn.id2551516" href="#id2551516" class="para">8</a>] </sup>
Detecting and fixing an error with ECC "on the fly" and not going the further
step and reassigning the block in question may explain why some disks have
large numbers in their read error counter log. Various worried users have
reported large numbers in the "errors corrected without substantial delay"
counter field which is in the "Errors corrected by ECC fast" column in
the <span class="command"><strong>smartctl -l error</strong></span> output.
</p></div><div class="footnote"><p><sup>[<a name="ftn.id2551535" href="#id2551535" class="para">9</a>] </sup>
Often disks inside a hardware RAID have the ARRE and AWRE bits
cleared (disabled) so the RAID controller can do things manually or flag
the disk for replacement.
</p></div><div class="footnote"><p><sup>[<a name="ftn.id2551756" href="#id2551756" class="para">10</a>] </sup>
In this case the corruption was manufactured by using the WRITE LONG
SCSI command. See <span class="command"><strong>sg_write_long</strong></span> in sg3_utils.
</p></div><div class="footnote"><p><sup>[<a name="ftn.id2551874" href="#id2551874" class="para">11</a>] </sup>
Most window managers have a handy calculator that will do hex to
decimal conversions. More work may be needed at the file system level,
</p></div></div></div></body></html>