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openSUSE:Step:15-SP1
qemu-linux-user.18973
0151-qcow2-Options-documentation-fixes.patch
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File 0151-qcow2-Options-documentation-fixes.patch of Package qemu-linux-user.18973
From: Leonid Bloch <lbloch@janustech.com> Date: Wed, 26 Sep 2018 19:04:39 +0300 Subject: qcow2: Options' documentation fixes Include-If: %if 0%{?suse_version} == 1315 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> (cherry picked from commit 40fb215d483ce510e211b843352288894eb13285) [LM: BSC#1139926] Signed-off-by: Lin Ma <lma@suse.com> --- docs/qcow2-cache.txt | 21 ++++++++++++++------- 1 file changed, 14 insertions(+), 7 deletions(-) diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt index b607b0a0f4bdbf4a075ce9800062..d87593dc2fb1f863c8b08dd2fe61 100644 --- a/docs/qcow2-cache.txt +++ b/docs/qcow2-cache.txt @@ -79,14 +79,14 @@ Choosing the right cache sizes In order to choose the cache sizes we need to know how they relate to the amount of allocated space. -The amount of virtual disk that can be mapped by the L2 and refcount +The part of the virtual disk that can be mapped by the L2 and refcount caches (in bytes) is: disk_size = l2_cache_size * cluster_size / 8 disk_size = refcount_cache_size * cluster_size * 8 / refcount_bits With the default values for cluster_size (64KB) and refcount_bits -(16), that is +(16), this becomes: disk_size = l2_cache_size * 8192 disk_size = refcount_cache_size * 32768 @@ -97,12 +97,16 @@ need: l2_cache_size = disk_size_GB * 131072 refcount_cache_size = disk_size_GB * 32768 -QEMU has a default L2 cache of 1MB (1048576 bytes) and a refcount -cache of 256KB (262144 bytes), so using the formulas we've just seen -we have +For example, 1MB of L2 cache is needed to cover every 8 GB of the virtual +image size (given that the default cluster size is used): - 1048576 / 131072 = 8 GB of virtual disk covered by that cache - 262144 / 32768 = 8 GB + 8 GB / 8192 = 1 MB + +The refcount cache is 4 times the cluster size by default. With the default +cluster size of 64 KB, it is 256 KB (262144 bytes). This is sufficient for +8 GB of image size: + + 262144 * 32768 = 8 GB How to configure the cache sizes @@ -130,6 +134,9 @@ There are a few things that need to be taken into account: memory as possible to the L2 cache before increasing the refcount cache size. + - At most two of "l2-cache-size", "refcount-cache-size", and "cache-size" + can be set simultaneously. + 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
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