From e3455ccc52d50010fdbd1940d7efebd28c65f3ad Mon Sep 17 00:00:00 2001

From: Kevin Wolf <kwolf@redhat.com>

Date: Thu, 6 Dec 2018 17:12:27 +0000

Subject: [PATCH 02/15] docs: Document the new default sizes of the qcow2

 caches

RH-Author: Kevin Wolf <kwolf@redhat.com>

Message-id: <20181206171240.5674-3-kwolf@redhat.com>

Patchwork-id: 83297

O-Subject: [RHEL-8.0 qemu-kvm PATCH 02/15] docs: Document the new default sizes of the qcow2 caches

Bugzilla: 1656507

RH-Acked-by: Max Reitz <mreitz@redhat.com>

RH-Acked-by: John Snow <jsnow@redhat.com>

RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

From: Alberto Garcia <berto@igalia.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>

(cherry picked from commit 603790ef3aec6a19b1c095188a1d2171934a27de)

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>

---

 docs/qcow2-cache.txt | 33 ++++++++++++++++-----------------

 1 file changed, 16 insertions(+), 17 deletions(-)

diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt

index 170191a..8a09a5c 100644

--- a/docs/qcow2-cache.txt

+++ b/docs/qcow2-cache.txt

@@ -116,31 +116,30 @@ 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

    (or the cache entry size: see "Using smaller cache sizes" below).

- - 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.

+Before QEMU 2.12 the refcount cache had a default size of 1/4 of the

+L2 cache size. This resulted in unnecessarily large caches, so now the

+refcount cache is as small as possible unless overridden by the user.

 Using smaller cache entries

-- 

1.8.3.1