Subcluster allocation in qcow2 is implemented by extending the
existing L2 table entries and adding additional information to
indicate the allocation status of each subcluster.
This patch documents the changes to the qcow2 format and how they
affect the calculation of the L2 cache size.
Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <5199f2e1c717bcaa58b48142c9062b803145ff7f.1594396418.git.berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
QEMU 2.12 (commit 1221fe6f63) introduced
a new setting called l2-cache-entry-size that allows making entries on
the qcow2 L2 cache smaller than the cluster size.
I have been performing several tests with different cluster and entry
sizes and all of them show that reducing the entry size (aka L2 slice)
consistently improves I/O performance, notably during random I/O (all
tests done with sequential I/O show similar results). This is to be
expected because loading and evicting an L2 slice is more expensive
the larger the slice is.
Here are some numbers on fully populated 40GB qcow2 images. The
rightmost column represents the maximum L2 cache size in both cases.
Cluster size = 64 KB
|-------------+--------------+--------------+--------------|
| | 1MB L2 cache | 3MB L2 cache | 5MB L2 cache |
|-------------+--------------+--------------+--------------|
| 4KB slices | 6545 IOPS | 12045 IOPS | 55680 IOPS |
| 16KB slices | 5177 IOPS | 9798 IOPS | 56278 IOPS |
| 64KB slices | 2718 IOPS | 5326 IOPS | 57355 IOPS |
|-------------+--------------+--------------+--------------|
Cluster size = 256 KB
|--------------+----------------+--------------+-----------------|
| | 512KB L2 cache | 1MB L2 cache | 1280KB L2 cache |
|--------------+----------------+--------------+-----------------|
| 4KB slices | 8539 IOPS | 21071 IOPS | 55417 IOPS |
| 64KB slices | 3598 IOPS | 9772 IOPS | 57687 IOPS |
| 256KB slices | 1415 IOPS | 4120 IOPS | 58001 IOPS |
|--------------+----------------+--------------+-----------------|
As can be seen in the numbers, the only exception to the rule is when
the cache is large enough to hold all L2 tables. This is also to be
expected because in this case no cache entry is ever evicted so
reducing its size doesn't bring any benefit.
This patch sets the default L2 cache entry size to 4KB except when the
cache is large enough for the whole disk.
Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Fixing cache-clean-interval documentation following the recent change to
a default of 600 seconds on supported plarforms (only Linux currently).
Signed-off-by: Leonid Bloch <lbloch@janustech.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
The default cache-clean-interval is set to 10 minutes, in order to lower
the overhead of the qcow2 caches (before the default was 0, i.e.
disabled).
* For non-Linux platforms the default is kept at 0, because
cache-clean-interval is not supported there yet.
Signed-off-by: Leonid Bloch <lbloch@janustech.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
The upper limit on the L2 cache size is increased from 1 MB to 32 MB
on Linux platforms, and to 8 MB on other platforms (this difference is
caused by the ability to set intervals for cache cleaning on Linux
platforms only).
This is done in order to allow default full coverage with the L2 cache
for images of up to 256 GB in size (was 8 GB). Note, that only the
needed amount to cover the full image is allocated. The value which is
changed here is just the upper limit on the L2 cache size, beyond which
it will not grow, even if the size of the image will require it to.
Signed-off-by: Leonid Bloch <lbloch@janustech.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Sufficient L2 cache can noticeably improve the performance when using
large images with frequent I/O.
Previously, unless 'cache-size' was specified and was large enough, the
L2 cache was set to a certain size without taking the virtual image size
into account.
Now, the L2 cache assignment is aware of the virtual size of the image,
and will cover the entire image, unless the cache size needed for that is
larger than a certain maximum. This maximum is set to 1 MB by default
(enough to cover an 8 GB image with the default cluster size) but can
be increased or decreased using the 'l2-cache-size' option. This option
was previously documented as the *maximum* L2 cache size, and this patch
makes it behave as such, instead of as a constant size. Also, the
existing option 'cache-size' can limit the sum of both L2 and refcount
caches, as previously.
Signed-off-by: Leonid Bloch <lbloch@janustech.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Leonid Bloch <lbloch@janustech.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
We have just reduced the refcount cache size to the minimum unless
the user explicitly requests a larger one, so we have to update the
documentation to reflect this change.
Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: c5f0bde23558dd9d33b21fffc76ac9953cc19c56.1523968389.git.berto@igalia.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
With the move of some docs to docs/interop on d59157ea05,
a reference path was not updated.
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Make it clear that having Linux is a hard requirement for this
feature.
Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
QEMU has options to configure the size of the L2 and refcount
caches for the qcow2 format. However, choosing the right sizes for
a particular disk image is not a straightforward operation since
the ratio between the cache size and the allocated disk space is
not obvious and depends on the size of the cluster and the refcount
entries.
This document attempts to give an overview of both caches and how to
configure their sizes.
Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 55de928e139b1ba3f3d40fe9c6c88f30b1f36410.1438690126.git.berto@igalia.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
