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charset="utf-8" Signed-off-by: Leonid Bloch --- docs/qcow2-cache.txt | 20 +++++++++++++------- qemu-options.hx | 9 ++++++--- 2 files changed, 19 insertions(+), 10 deletions(-) diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt index 8a09a5cc5f..2326db01b9 100644 --- a/docs/qcow2-cache.txt +++ b/docs/qcow2-cache.txt @@ -77,7 +77,7 @@ aforementioned L2 cache, and its size can also be configu= red. 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 the allocated space. =20 The amount of virtual disk that can be mapped by the L2 and refcount caches (in bytes) is: @@ -86,7 +86,7 @@ caches (in bytes) is: disk_size =3D refcount_cache_size * cluster_size * 8 / refcount_bits =20 With the default values for cluster_size (64KB) and refcount_bits -(16), that is +(16), this becomes: =20 disk_size =3D l2_cache_size * 8192 disk_size =3D refcount_cache_size * 32768 @@ -97,12 +97,15 @@ need: l2_cache_size =3D disk_size_GB * 131072 refcount_cache_size =3D disk_size_GB * 32768 =20 -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): =20 - 1048576 / 131072 =3D 8 GB of virtual disk covered by that cache - 262144 / 32768 =3D 8 GB + 8 * 131072 =3D 1 MB + +A default refcount cache is 4 times the cluster size, which defaults to +256 KB (262144 bytes). This is sufficient for 8 GB of image size: + + 262144 / 32768 =3D 8 GB =20 =20 How to configure the cache sizes @@ -130,6 +133,9 @@ There are a few things that need to be taken into accou= nt: memory as possible to the L2 cache before increasing the refcount cache size. =20 + - 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 diff --git a/qemu-options.hx b/qemu-options.hx index b1bf0f485f..f6804758d3 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -752,15 +752,18 @@ image file) =20 @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) =20 @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, whic= hever +is larger; otherwise, as large as possible or needed within the cache-size, +while permitting the requested or the minimal refcount cache size) =20 @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 par= t of +it which is not used for the L2 cache) =20 @item cache-clean-interval Clean unused entries in the L2 and refcount caches. The interval is in sec= onds. --=20 2.17.1