diff options
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/qcow2-cache.txt | 59 |
1 files changed, 37 insertions, 22 deletions
diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt index 8a09a5cc5f..c459bf5dd3 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 @@ -121,8 +125,15 @@ There are a few things that need to be taken into account: - Both caches must have a size that is a multiple of the cluster size (or the cache entry size: see "Using smaller cache sizes" below). - - The default L2 cache size is 8 clusters or 1MB (whichever is more), - and the minimum is 2 clusters (or 2 cache entries, see below). + - The maximum L2 cache size is 32 MB by default on Linux platforms (enough + for full coverage of 256 GB images, with the default cluster size). This + value can be modified using the "l2-cache-size" option. QEMU will not use + more memory than needed to hold all of the image's L2 tables, regardless + of this max. value. + On non-Linux platforms the maximal value is smaller by default (8 MB) and + this difference stems from the fact that on Linux the cache can be cleared + periodically if needed, using the "cache-clean-interval" option (see below). + The minimal L2 cache size is 2 clusters (or 2 cache entries, see below). - The default (and minimum) refcount cache size is 4 clusters. @@ -130,6 +141,9 @@ 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 @@ -177,9 +191,10 @@ Some things to take into account: always uses the cluster size as the entry size. - If the L2 cache is big enough to hold all of the image's L2 tables - (as explained in the "Choosing the right cache sizes" section - earlier in this document) then none of this is necessary and you - can omit the "l2-cache-entry-size" parameter altogether. + (as explained in the "Choosing the right cache sizes" and "How to + configure the cache sizes" sections in this document) then none of + this is necessary and you can omit the "l2-cache-entry-size" + parameter altogether. Reducing the memory usage @@ -187,18 +202,18 @@ Reducing the memory usage It is possible to clean unused cache entries in order to reduce the memory usage during periods of low I/O activity. -The parameter "cache-clean-interval" defines an interval (in seconds). -All cache entries that haven't been accessed during that interval are -removed from memory. +The parameter "cache-clean-interval" defines an interval (in seconds), +after which all the cache entries that haven't been accessed during the +interval are removed from memory. Setting this parameter to 0 disables this +feature. -This example removes all unused cache entries every 15 minutes: +The following example removes all unused cache entries every 15 minutes: -drive file=hd.qcow2,cache-clean-interval=900 -If unset, the default value for this parameter is 0 and it disables -this feature. +If unset, the default value for this parameter is 600 on platforms which +support this functionality, and is 0 (disabled) on other platforms. -Note that this functionality currently relies on the MADV_DONTNEED -argument for madvise() to actually free the memory. This is a -Linux-specific feature, so cache-clean-interval is not supported in -other systems. +This functionality currently relies on the MADV_DONTNEED argument for +madvise() to actually free the memory. This is a Linux-specific feature, +so cache-clean-interval is not supported on other systems. |