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From a608710425e4de7446a0ca5591f00751af971b0a 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 17:12:30 +0000
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Subject: [PATCH 05/15] qcow2: Options' documentation fixes
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RH-Author: Kevin Wolf <kwolf@redhat.com>
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Message-id: <20181206171240.5674-6-kwolf@redhat.com>
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Patchwork-id: 83285
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O-Subject: [RHEL-8.0 qemu-kvm PATCH 05/15] qcow2: Options' documentation fixes
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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: Leonid Bloch <lbloch@janustech.com>
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Signed-off-by: Leonid Bloch <lbloch@janustech.com>
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Reviewed-by: Alberto Garcia <berto@igalia.com>
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Reviewed-by: Kevin Wolf <kwolf@redhat.com>
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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(cherry picked from commit 40fb215d483ce510e211b843352288894eb13285)
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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 | 21 ++++++++++++++-------
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qemu-options.hx | 9 ++++++---
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2 files changed, 20 insertions(+), 10 deletions(-)
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diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
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index 8a09a5c..7e28b41 100644
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--- a/docs/qcow2-cache.txt
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+++ b/docs/qcow2-cache.txt
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@@ -79,14 +79,14 @@ Choosing the right cache sizes
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In order to choose the cache sizes we need to know how they relate to
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the amount of allocated space.
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-The amount of virtual disk that can be mapped by the L2 and refcount
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+The part of the virtual disk that can be mapped by the L2 and refcount
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caches (in bytes) is:
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disk_size = l2_cache_size * cluster_size / 8
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disk_size = refcount_cache_size * cluster_size * 8 / refcount_bits
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With the default values for cluster_size (64KB) and refcount_bits
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-(16), that is
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+(16), this becomes:
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disk_size = l2_cache_size * 8192
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disk_size = refcount_cache_size * 32768
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@@ -97,12 +97,16 @@ need:
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l2_cache_size = disk_size_GB * 131072
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refcount_cache_size = disk_size_GB * 32768
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-QEMU has a default L2 cache of 1MB (1048576 bytes) and a refcount
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-cache of 256KB (262144 bytes), so using the formulas we've just seen
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-we have
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+For example, 1MB of L2 cache is needed to cover every 8 GB of the virtual
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+image size (given that the default cluster size is used):
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- 1048576 / 131072 = 8 GB of virtual disk covered by that cache
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- 262144 / 32768 = 8 GB
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+ 8 GB / 8192 = 1 MB
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+
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+The refcount cache is 4 times the cluster size by default. With the default
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+cluster size of 64 KB, it is 256 KB (262144 bytes). This is sufficient for
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+8 GB of image size:
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+
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+ 262144 * 32768 = 8 GB
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How to configure the cache sizes
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@@ -130,6 +134,9 @@ There are a few things that need to be taken into account:
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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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+ - At most two of "l2-cache-size", "refcount-cache-size", and "cache-size"
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+ can be set simultaneously.
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+
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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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diff --git a/qemu-options.hx b/qemu-options.hx
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index 2e05112..5e15d6f 100644
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--- a/qemu-options.hx
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+++ b/qemu-options.hx
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@@ -752,15 +752,18 @@ image file)
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@item cache-size
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The maximum total size of the L2 table and refcount block caches in bytes
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-(default: 1048576 bytes or 8 clusters, whichever is larger)
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+(default: the sum of l2-cache-size and refcount-cache-size)
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@item l2-cache-size
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The maximum size of the L2 table cache in bytes
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-(default: 4/5 of the total cache size)
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+(default: if cache-size is not defined - 1048576 bytes or 8 clusters, whichever
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+is larger; otherwise, as large as possible or needed within the cache-size,
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+while permitting the requested or the minimal refcount cache size)
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@item refcount-cache-size
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The maximum size of the refcount block cache in bytes
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-(default: 1/5 of the total cache size)
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+(default: 4 times the cluster size; or if cache-size is specified, the part of
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+it which is not used for the L2 cache)
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@item cache-clean-interval
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Clean unused entries in the L2 and refcount caches. The interval is in seconds.
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--
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1.8.3.1
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