=encoding utf8

=head1 НАЗВА

guestfs-performance - engineering libguestfs for greatest performance

=head1 ОПИС

This page documents how to get the greatest performance out of libguestfs,
especially when you expect to use libguestfs to manipulate thousands of
virtual machines or disk images.

Three main areas are covered. Libguestfs runs an appliance (a small Linux
distribution) inside qemu/KVM.  The first two areas are: minimizing the time
taken to start this appliance, and the number of times the appliance has to
be started.  The third area is shortening the time taken for inspection of
VMs.

=head1 BASELINE MEASUREMENTS

Before making changes to how you use libguestfs, take baseline measurements.

=head2 BASELINE: STARTING THE APPLIANCE

On an unloaded machine, time how long it takes to start up the appliance:

 time guestfish -a /dev/null run

Run this command several times in a row and discard the first few runs, so
that you are measuring a typical "hot cache" case.

=head3 Пояснення

This command starts up the libguestfs appliance on a null disk, and then
immediately shuts it down.  The first time you run the command, it will
create an appliance and cache it (usually under C</var/tmp/.guestfs-*>).
Subsequent runs should reuse the cached appliance.

=head3 Expected results

You should expect to be getting times under 6 seconds.  If the times you see
on an unloaded machine are above this, then see the section
L</TROUBLESHOOTING POOR PERFORMANCE> below.

=head2 BASELINE: PERFORMING INSPECTION OF A GUEST

For this test you will need an unloaded machine and at least one real guest
or disk image.  If you are planning to use libguestfs against only X guests
(eg. X = Windows), then using an X guest here would be most appropriate.  If
you are planning to run libguestfs against a mix of guests, then use a mix
of guests for testing here.

Time how long it takes to perform inspection and mount the disks of the
guest.  Use the first command if you will be using disk images, and the
second command if you will be using libvirt.

 time guestfish --ro -a disk.img -i exit

 time guestfish --ro -d GuestName -i exit

Run the command several times in a row and discard the first few runs, so
that you are measuring a typical "hot cache" case.

=head3 Пояснення

This command starts up the libguestfs appliance on the named disk image or
libvirt guest, performs libguestfs inspection on it (see
L<guestfs(3)/INSPECTION>), mounts the guest's disks, then discards all these
results and shuts down.

The first time you run the command, it will create an appliance and cache it
(usually under C</var/tmp/.guestfs-*>).  Subsequent runs should reuse the
cached appliance.

=head3 Expected results

You should expect times which are E<le> 5 seconds greater than measured in
the first baseline test above.  (For example, if the first baseline test ran
in 5 seconds, then this test should run in E<le> 10 seconds).

=head1 UNDERSTANDING THE APPLIANCE AND WHEN IT IS BUILT/CACHED

The first time you use libguestfs, it will build and cache an appliance.
This is usually in C</var/tmp/.guestfs-*>, unless you have set C<$TMPDIR> in
which case it will be under that temporary directory.

For more information about how the appliance is constructed, see
L<febootstrap(8)/SUPERMIN APPLIANCES>.

Every time libguestfs runs it will check that no host files used by the
appliance have changed.  If any have, then the appliance is rebuilt.  This
usually happens when a package is installed or updated on the host
(eg. using programs like C<yum> or C<apt-get>).  The reason for
reconstructing the appliance is security: the new program that has been
installed might contain a security fix, and so we want to include the fixed
program in the appliance automatically.

These are the performance implications:

=over 4

=item *

The process of building (or rebuilding) the cached appliance is slow, and
you can avoid this happening by using a fixed appliance (see below).

=item *

If not using a fixed appliance, be aware that updating software on the host
will cause a one time rebuild of the appliance.

=item *

C</var/tmp> (or C<$TMPDIR>) should be on a fast disk, and have plenty of
space for the appliance.

=back

=head1 USING A FIXED APPLIANCE

To fully control when the appliance is built, you can build a fixed
appliance.  This appliance can and should be stored on a fast, local disk.

To build the appliance, run the command:

 libguestfs-make-fixed-appliance <directory>

replacing C<E<lt>directoryE<gt>> with the name of a directory where the
appliance will be stored (normally you would name a subdirectory, for
example: C</usr/local/lib/guestfs/appliance> or C</dev/shm/appliance>).

Then set C<$LIBGUESTFS_PATH> (and ensure this environment variable is set in
your libguestfs program), or modify your program so it calls
C<guestfs_set_path>.  For example:

 export LIBGUESTFS_PATH=/usr/local/lib/guestfs/appliance

Now you can run libguestfs programs, virt tools, guestfish etc. as normal.
The programs will use your fixed appliance, and will not ever build,
rebuild, or cache their own appliance.

(For detailed information on this subject, see:
L<libguestfs-make-fixed-appliance(1)>).

=head2 PERFORMANCE OF THE FIXED APPLIANCE

In our testing we did not find that using a fixed appliance gave any
measurable performance benefit, even when the appliance was located in
memory (ie. on C</dev/shm>).  However there are three points to consider:

=over 4

=item 1.

Using a fixed appliance stops libguestfs from ever rebuilding the appliance,
meaning that libguestfs will have more predictable start-up times.

=item 2.

By default libguestfs (or rather, L<febootstrap-supermin-helper(8)>)
searches over the root filesystem to find out if any host files have changed
and if it needs to rebuild the appliance.  If these files are not cached and
the root filesystem is on an HDD, then this generates lots of seeks.  Using
a fixed appliance avoids all this.

=item 3.

The appliance is loaded on demand.  A simple test such as:

 time guestfish -a /dev/null run

does not load very much of the appliance.  A real libguestfs program using
complicated API calls would demand-load a lot more of the appliance.  Being
able to store the appliance in a specified location makes the performance
more predictable.

=back

=head1 REDUCING THE NUMBER OF TIMES THE APPLIANCE IS LAUNCHED

By far the most effective, though not always the simplest way to get good
performance is to ensure that the appliance is launched the minimum number
of times.  This will probably involve changing your libguestfs application.

Try to call C<guestfs_launch> at most once per virtual machine.

Instead of using a separate instance of L<guestfish(1)> to make a series of
changes to the same guest, use a single instance of guestfish and/or use the
guestfish I<--listen> option.

Consider writing your program as a daemon which holds a guest open while
making a series of changes.  Or marshal all the operations you want to
perform before opening the guest.

You can also try adding disks from multiple guests to a single appliance.
Before trying this, note the following points:

=over 4

=item 1.

Adding multiple guests to one appliance is a security problem because it may
allow one guest to interfere with the disks of another guest.  Only do it if
you trust all the guests, or if you can group guests by trust.

=item 2.

In current qemu, there is a limit of around 26 disks that can be added to
the appliance.  In future versions of qemu (and hence libguestfs)  we hope
to lift this limit.

=item 3.

Using libguestfs this way is complicated.  Disks can have unexpected
interactions: for example, if two guests use the same UUID for a filesystem
(because they were cloned), or have volume groups with the same name (but
see C<guestfs_lvm_set_filter>).

=back

L<virt-df(1)> adds multiple disks by default, so the source code for this
program would be a good place to start.

=head1 SHORTENING THE TIME TAKEN FOR INSPECTION OF VMs

The main advice is obvious: Do not perform inspection (which is expensive)
unless you need the results.

If you previously performed inspection on the guest, then it may be safe to
cache and reuse the results from last time.

Some disks don't need to be inspected at all: for example, if you are
creating a disk image, or if the disk image is not a VM, or if the disk
image has a known layout.

Even when basic inspection (C<guestfs_inspect_os>) is required, auxiliary
inspection operations may be avoided:

=over 4

=item *

Mounting disks is only necessary to get further filesystem information.

=item *

Listing applications (C<guestfs_inspect_list_applications>) is an expensive
operation on Linux, but almost free on Windows.

=item *

Generating a guest icon (C<guestfs_inspect_get_icon>) is cheap on Linux but
expensive on Windows.

=back

=head1 TROUBLESHOOTING POOR PERFORMANCE

=head2 ENSURE HARDWARE VIRTUALIZATION IS AVAILABLE

Use C</proc/cpuinfo> and this page:

http://virt-tools.org/learning/check-hardware-virt/

to ensure that hardware virtualization is available.  Note that you may need
to enable it in your BIOS.

Hardware virt is not usually available inside VMs, and libguestfs will run
slowly inside another virtual machine whatever you do.  Nested
virtualization does not work well in our experience, and is certainly no
substitute for running libguestfs on baremetal.

=head2 ENSURE KVM IS AVAILABLE

Ensure that KVM is enabled and available to the user that will run
libguestfs.  It should be safe to set 0666 permissions on C</dev/kvm> and
most distributions now do this.

=head2 PROCESSORS TO AVOID

Avoid processors that don't have hardware virtualization, and some
processors which are simply very slow (AMD Geode being a great example).

=head1 DETAILED TIMINGS USING SYSTEMTAP

You can use SystemTap (L<stap(1)>) to get detailed timings from libguestfs
programs.

Save the following script as C<time.stap>:

 global last;
 
 function display_time () {
       now = gettimeofday_us ();
       delta = 0;
       if (last > 0)
             delta = now - last;
       last = now;
 
       printf ("%d (+%d):", now, delta);
 }
 
 probe begin {
       last = 0;
       printf ("ready\n");
 }
 
 /* Display all calls to static markers. */
 probe process("/usr/lib*/libguestfs.so.0")
           .provider("guestfs").mark("*") ? {
       display_time();
       printf ("\t%s %s\n", $$name, $$parms);
 }
 
 /* Display all calls to guestfs_* functions. */
 probe process("/usr/lib*/libguestfs.so.0")
           .function("guestfs_[a-z]*") ? {
       display_time();
       printf ("\t%s %s\n", probefunc(), $$parms);
 }

Run it as root in one window:

 # stap time.stap
 ready

It prints "ready" when SystemTap has loaded the program.  Run your
libguestfs program, guestfish or a virt tool in another window.  For
example:

 $ guestfish -a /dev/null run

In the stap window you will see a large amount of output, with the time
taken for each step shown (microseconds in parenthesis).  For example:

 xxxx (+0):	guestfs_create 
 xxxx (+29):	guestfs_set_pgroup g=0x17a9de0 pgroup=0x1
 xxxx (+9):	guestfs_add_drive_opts_argv g=0x17a9de0 [...]
 xxxx (+8):	guestfs_safe_strdup g=0x17a9de0 str=0x7f8a153bed5d
 xxxx (+19):	guestfs_safe_malloc g=0x17a9de0 nbytes=0x38
 xxxx (+5):	guestfs_safe_strdup g=0x17a9de0 str=0x17a9f60
 xxxx (+10):	guestfs_launch g=0x17a9de0
 xxxx (+4):	launch_start 
 [etc]

You will need to consult, and even modify, the source to libguestfs to fully
understand the output.

=head1 ТАКОЖ ПЕРЕГЛЯНЬТЕ

L<febootstrap(8)>, L<febootstrap-supermin-helper(8)>, L<guestfish(1)>,
L<guestfs(3)>, L<guestfs-examples(3)>,
L<libguestfs-make-fixed-appliance(1)>, L<stap(1)>,
L<http://libguestfs.org/>.

=head1 АВТОРИ

Richard W.M. Jones (C<rjones at redhat dot com>)

=head1 АВТОРСЬКІ ПРАВА

Copyright (C) 2012 Red Hat Inc. L<http://libguestfs.org/>

This library is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by the
Free Software Foundation; either version 2 of the License, or (at your
option) any later version.

Ця бібліотека поширюється у сподіванні, що вона буде корисною, але БЕЗ
БУДЬ-ЯКИХ ГАРАНТІЙНИХ ЗОБОВ’ЯЗАНЬ; навіть без очевидної гарантії
ПРАЦЕЗДАТНОСТІ або ПРИДАТНОСТІ ДЛЯ ВИКОРИСТАННЯ З ПЕВНОЮ МЕТОЮ. Докладніше
про це можна дізнатися з GNU General Public License.

Ви маєте отримати копію GNU Lesser General Public License разом з цією
бібліотекою; якщо це не так, повідомте про факт за адресою Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
USA