File 0151-qcow2-Options-documentation-fixes.patch of Package qemu.20749

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File 0151-qcow2-Options-documentation-fixes.patch of Package qemu.20749

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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File 0151-qcow2-Options-documentation-fixes.patch of Package qemu.20749

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