NAME¶
guestfs-performance - engineering libguestfs for greatest performance
DESCRIPTION¶
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.
BASELINE MEASUREMENTS¶
Before making changes to how you use libguestfs, take baseline measurements.
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.
Explanation
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 "/var/tmp/.guestfs-*").
Subsequent runs should reuse the cached appliance.
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 "TROUBLESHOOTING
POOR PERFORMANCE" below.
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.
Explanation
This command starts up the libguestfs appliance on the named disk image or
libvirt guest, performs libguestfs inspection on it (see
"INSPECTION" in
guestfs(3)), 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 "/var/tmp/.guestfs-*"). Subsequent runs should reuse
the cached appliance.
Expected results
You should expect times which are ≤ 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 ≤ 10 seconds).
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 "/var/tmp/.guestfs-*", unless you have set $TMPDIR in
which case it will be under that temporary directory.
For more information about how the appliance is constructed, see "SUPERMIN
APPLIANCES" in
febootstrap(8).
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 "yum" or "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:
- •
- The process of building (or rebuilding) the cached
appliance is slow, and you can avoid this happening by using a fixed
appliance (see below).
- •
- If not using a fixed appliance, be aware that updating
software on the host will cause a one time rebuild of the appliance.
- •
- "/var/tmp" (or $TMPDIR) should be on a fast disk,
and have plenty of space for the appliance.
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 "<directory>" with the name of a directory where the
appliance will be stored (normally you would name a subdirectory, for example:
"/usr/local/lib/guestfs/appliance" or
"/dev/shm/appliance").
Then set $LIBGUESTFS_PATH (and ensure this environment variable is set in your
libguestfs program), or modify your program so it calls
"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:
libguestfs-make-fixed-appliance(1)).
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
"/dev/shm"). However there are three points to consider:
- 1.
- Using a fixed appliance stops libguestfs from ever
rebuilding the appliance, meaning that libguestfs will have more
predictable start-up times.
- 2.
- By default libguestfs (or rather,
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.
- 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.
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 "guestfs_launch" at most once per virtual machine.
Instead of using a separate instance of
guestfish(1) to make a series of
changes to the same guest, use a single instance of guestfish and/or use the
guestfish
--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:
- 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.
- 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.
- 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 "guestfs_lvm_set_filter").
virt-df(1) adds multiple disks by default, so the source code for this
program would be a good place to start.
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 ("guestfs_inspect_os") is required,
auxiliary inspection operations may be avoided:
- •
- Mounting disks is only necessary to get further filesystem
information.
- •
- Listing applications
("guestfs_inspect_list_applications") is an expensive operation
on Linux, but almost free on Windows.
- •
- Generating a guest icon
("guestfs_inspect_get_icon") is cheap on Linux but expensive on
Windows.
ENSURE HARDWARE VIRTUALIZATION IS AVAILABLE¶
Use "/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.
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 "/dev/kvm" and most
distributions now do this.
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).
DETAILED TIMINGS USING SYSTEMTAP¶
You can use SystemTap (
stap(1)) to get detailed timings from libguestfs
programs.
Save the following script as "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.
SEE ALSO¶
febootstrap(8),
febootstrap-supermin-helper(8),
guestfish(1),
guestfs(3),
guestfs-examples(3),
libguestfs-make-fixed-appliance(1),
stap(1),
<
http://libguestfs.org/>.
AUTHORS¶
Richard W.M. Jones ("rjones at redhat dot com")
COPYRIGHT¶
Copyright (C) 2012 Red Hat Inc. <
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.
This library is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more
details.
You should have received a copy of the GNU Lesser General Public License along
with this library; if not, write to the Free Software Foundation, Inc., 51
Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
