diff options
| -rw-r--r-- | docs/qcow2-cache.txt | 21 | ||||
| -rw-r--r-- | qemu-options.hx | 9 |
2 files changed, 20 insertions, 10 deletions
diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt index 8a09a5c..7e28b41 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 diff --git a/qemu-options.hx b/qemu-options.hx index a642ad2..2db6247 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -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. |