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From e3455ccc52d50010fdbd1940d7efebd28c65f3ad Mon Sep 17 00:00:00 2001
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From: Kevin Wolf <kwolf@redhat.com>
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Date: Thu, 6 Dec 2018 17:12:27 +0000
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Subject: [PATCH 02/15] docs: Document the new default sizes of the qcow2
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caches
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RH-Author: Kevin Wolf <kwolf@redhat.com>
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Message-id: <20181206171240.5674-3-kwolf@redhat.com>
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Patchwork-id: 83297
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O-Subject: [RHEL-8.0 qemu-kvm PATCH 02/15] docs: Document the new default sizes of the qcow2 caches
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Bugzilla: 1656507
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RH-Acked-by: Max Reitz <mreitz@redhat.com>
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RH-Acked-by: John Snow <jsnow@redhat.com>
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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From: Alberto Garcia <berto@igalia.com>
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We have just reduced the refcount cache size to the minimum unless
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the user explicitly requests a larger one, so we have to update the
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documentation to reflect this change.
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Signed-off-by: Alberto Garcia <berto@igalia.com>
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Message-id: c5f0bde23558dd9d33b21fffc76ac9953cc19c56.1523968389.git.berto@igalia.com
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Reviewed-by: Eric Blake <eblake@redhat.com>
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Signed-off-by: Max Reitz <mreitz@redhat.com>
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(cherry picked from commit 603790ef3aec6a19b1c095188a1d2171934a27de)
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
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---
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docs/qcow2-cache.txt | 33 ++++++++++++++++-----------------
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1 file changed, 16 insertions(+), 17 deletions(-)
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diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
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index 170191a..8a09a5c 100644
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--- a/docs/qcow2-cache.txt
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+++ b/docs/qcow2-cache.txt
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@@ -116,31 +116,30 @@ There are three options available, and all of them take bytes:
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"refcount-cache-size": maximum size of the refcount block cache
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"cache-size": maximum size of both caches combined
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-There are two things that need to be taken into account:
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+There are a few things that need to be taken into account:
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- Both caches must have a size that is a multiple of the cluster size
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(or the cache entry size: see "Using smaller cache sizes" below).
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- - If you only set one of the options above, QEMU will automatically
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- adjust the others so that the L2 cache is 4 times bigger than the
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- refcount cache.
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+ - The default L2 cache size is 8 clusters or 1MB (whichever is more),
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+ and the minimum is 2 clusters (or 2 cache entries, see below).
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-This means that these options are equivalent:
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+ - The default (and minimum) refcount cache size is 4 clusters.
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- -drive file=hd.qcow2,l2-cache-size=2097152
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- -drive file=hd.qcow2,refcount-cache-size=524288
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- -drive file=hd.qcow2,cache-size=2621440
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+ - If only "cache-size" is specified then QEMU will assign as much
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+ memory as possible to the L2 cache before increasing the refcount
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+ cache size.
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-The reason for this 1/4 ratio is to ensure that both caches cover the
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-same amount of disk space. Note however that this is only valid with
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-the default value of refcount_bits (16). If you are using a different
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-value you might want to calculate both cache sizes yourself since QEMU
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-will always use the same 1/4 ratio.
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+Unlike L2 tables, refcount blocks are not used during normal I/O but
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+only during allocations and internal snapshots. In most cases they are
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+accessed sequentially (even during random guest I/O) so increasing the
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+refcount cache size won't have any measurable effect in performance
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+(this can change if you are using internal snapshots, so you may want
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+to think about increasing the cache size if you use them heavily).
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-It's also worth mentioning that there's no strict need for both caches
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-to cover the same amount of disk space. The refcount cache is used
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-much less often than the L2 cache, so it's perfectly reasonable to
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-keep it small.
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+Before QEMU 2.12 the refcount cache had a default size of 1/4 of the
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+L2 cache size. This resulted in unnecessarily large caches, so now the
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+refcount cache is as small as possible unless overridden by the user.
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Using smaller cache entries
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--
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1.8.3.1
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