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qcow2: Options' documentation fixes

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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>
This commit is contained in:
Leonid Bloch 2018-09-26 19:04:39 +03:00 committed by Kevin Wolf
parent 543770bd2e
commit 40fb215d48
2 changed files with 20 additions and 10 deletions
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21
docs/qcow2-cache.txt
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@ -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

9
qemu-options.hx
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@ -732,15 +732,18 @@ image file)
@item cache-size
The maximum total size of the L2 table and refcount block caches in bytes
(default: 1048576 bytes or 8 clusters, whichever is larger)
(default: the sum of l2-cache-size and refcount-cache-size)
@item l2-cache-size
The maximum size of the L2 table cache in bytes
(default: 4/5 of the total cache size)
(default: if cache-size is not defined - 1048576 bytes or 8 clusters, whichever
is larger; otherwise, as large as possible or needed within the cache-size,
while permitting the requested or the minimal refcount cache size)
@item refcount-cache-size
The maximum size of the refcount block cache in bytes
(default: 1/5 of the total cache size)
(default: 4 times the cluster size; or if cache-size is specified, the part of
it which is not used for the L2 cache)
@item cache-clean-interval
Clean unused entries in the L2 and refcount caches. The interval is in seconds.