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Spew FAQ
=============
This is the Spew FAQ. Any suggestions/corrections/etc should be
directed to andrew.patterson@hp.com. The FAQ answers the following questions:
1) How do I get direct I/O working on RedHat Advanced Server 2.1?
2) Who came up with the names "Spew"?
3) How do I by-pass the buffer or disk cache to find the true
performance of a device?
1) How do I get direct I/O working on RedHat Advanced Server 2.1?
RHAS 2.1 has support in the kernel for direct I/O to a block device, but
it is not completely enabled. Applying the following patch will allow
direct I/O to any block device:
--- rhel2.lu3-test/fs/block_dev.c.orig 2004-07-27 16:48:20.000000000 -0600
+++ rhel2.lu3-test/fs/block_dev.c 2004-07-27 16:50:25.000000000 -0600
@@ -668,6 +668,7 @@
sync_page: block_sync_page,
prepare_write: blkdev_prepare_write,
commit_write: blkdev_commit_write,
+ direct_IO: blkdev_direct_IO,
};
struct file_operations def_blk_fops = {
2) Who came up with the name "Spew"?
Larry Fenske was the author of the original 20-line C version of Spew.
He also originated the name.
3) How do I by-pass the buffer or disk cache to find the true performance
of a device?
You can use the -s|--synchronous option for writes. This adds
the O_SYNC option when the file is opened. All writes will then
block until flushed to disk (Note that your device itself may
still cache).
Reads are a little more complicated. The O_SYNC option is
ignored for reads. On Linux, you can use spew's -d|--direct
option which will bypass the buffer cache for both reads and
writes. It will also disable read-ahead for reads. On other
operating systems (like HP-UX) or old versions of Linux (kernels
before 2.4.21 or so), you may have to use a special device
file. On old versions of Linux, you would use raw device files
(see raw(8)). On HP-UX, you would use the the /dev/rdsk/* device
files. I have no idea what to use on Windows.
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