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openSUSE:Step:15-SP2
qemu-testsuite.19805
0149-docs-Document-the-new-default-sizes.patch
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File 0149-docs-Document-the-new-default-sizes.patch of Package qemu-testsuite.19805
From: Alberto Garcia <berto@igalia.com> Date: Tue, 17 Apr 2018 15:37:05 +0300 Subject: docs: Document the new default sizes of the qcow2 caches Include-If: %if 0%{?suse_version} == 1315 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> (cherry picked from commit 603790ef3aec6a19b1c095188a1d2171934a27de) [LM: BSC#1139926] Signed-off-by: Lin Ma <lma@suse.com> --- docs/qcow2-cache.txt | 32 +++++++++++++++----------------- 1 file changed, 15 insertions(+), 17 deletions(-) diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt index b0571de4b826234fc15d56a4074c..b607b0a0f4bdbf4a075ce9800062 100644 --- a/docs/qcow2-cache.txt +++ b/docs/qcow2-cache.txt @@ -116,31 +116,29 @@ There are three options available, and all of them take bytes: "refcount-cache-size": maximum size of the refcount block cache "cache-size": maximum size of both caches combined -There are two things that need to be taken into account: +There are a few things that need to be taken into account: - Both caches must have a size that is a multiple of the cluster size. - - If you only set one of the options above, QEMU will automatically - adjust the others so that the L2 cache is 4 times bigger than the - refcount cache. + - The default L2 cache size is 8 clusters or 1MB (whichever is more), + and the minimum is 2 clusters (or 2 cache entries, see below). -This means that these options are equivalent: + - The default (and minimum) refcount cache size is 4 clusters. - -drive file=hd.qcow2,l2-cache-size=2097152 - -drive file=hd.qcow2,refcount-cache-size=524288 - -drive file=hd.qcow2,cache-size=2621440 + - If only "cache-size" is specified then QEMU will assign as much + memory as possible to the L2 cache before increasing the refcount + cache size. -The reason for this 1/4 ratio is to ensure that both caches cover the -same amount of disk space. Note however that this is only valid with -the default value of refcount_bits (16). If you are using a different -value you might want to calculate both cache sizes yourself since QEMU -will always use the same 1/4 ratio. +Unlike L2 tables, refcount blocks are not used during normal I/O but +only during allocations and internal snapshots. In most cases they are +accessed sequentially (even during random guest I/O) so increasing the +refcount cache size won't have any measurable effect in performance +(this can change if you are using internal snapshots, so you may want +to think about increasing the cache size if you use them heavily). -It's also worth mentioning that there's no strict need for both caches -to cover the same amount of disk space. The refcount cache is used -much less often than the L2 cache, so it's perfectly reasonable to -keep it small. +Now the refcount cache is as small as possible unless overridden by the +user. Reducing the memory usage
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