Sufficient L2 cache can noticeably improve the performance when using large images with frequent I/O.
Previously, the L2 cache was allocated without considering the image size, and an option existed to manually determine its size. Thus to achieve a full coverage of an image by the L2 cache (i.e. use more than the default value of MAX(1 MB, 8 clusters)), a user needed to calculate the required size manually, or with a script, and pass this value to the 'l2-cache-size' option. Now, the L2 cache is assigned taking the virtual image size into account, and will cover the entire image, unless the size needed for that is larger than a certain maximum. This maximum is set to 1 MB by default (enough to cover an 8 GB image with the default cluster size) but can be increased or decreased using the 'l2-cache-size' option. This option was previously documented as the *maximum* L2 cache size, and this patch makes it behave as such, instead of as a constant size. Also, the existing option 'cache-size' can limit the sum of both L2 and refcount caches, as previously. Signed-off-by: Leonid Bloch <lbl...@janustech.com> --- block/qcow2.c | 22 ++++++++++------------ block/qcow2.h | 4 +--- docs/qcow2-cache.txt | 13 ++++++++----- qemu-options.hx | 6 +++--- tests/qemu-iotests/137 | 1 - tests/qemu-iotests/137.out | 1 - 6 files changed, 22 insertions(+), 25 deletions(-) diff --git a/block/qcow2.c b/block/qcow2.c index 053242f94e..434fb89076 100644 --- a/block/qcow2.c +++ b/block/qcow2.c @@ -777,16 +777,19 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts *opts, uint64_t *refcount_cache_size, Error **errp) { BDRVQcow2State *s = bs->opaque; - uint64_t combined_cache_size; + uint64_t combined_cache_size, l2_cache_max_setting; bool l2_cache_size_set, refcount_cache_size_set, combined_cache_size_set; int min_refcount_cache = MIN_REFCOUNT_CACHE_SIZE * s->cluster_size; + uint64_t virtual_disk_size = bs->total_sectors * BDRV_SECTOR_SIZE; + uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8); combined_cache_size_set = qemu_opt_get(opts, QCOW2_OPT_CACHE_SIZE); l2_cache_size_set = qemu_opt_get(opts, QCOW2_OPT_L2_CACHE_SIZE); refcount_cache_size_set = qemu_opt_get(opts, QCOW2_OPT_REFCOUNT_CACHE_SIZE); combined_cache_size = qemu_opt_get_size(opts, QCOW2_OPT_CACHE_SIZE, 0); - *l2_cache_size = qemu_opt_get_size(opts, QCOW2_OPT_L2_CACHE_SIZE, 0); + l2_cache_max_setting = qemu_opt_get_size(opts, QCOW2_OPT_L2_CACHE_SIZE, + DEFAULT_L2_CACHE_MAX_SIZE); *refcount_cache_size = qemu_opt_get_size(opts, QCOW2_OPT_REFCOUNT_CACHE_SIZE, 0); @@ -794,13 +797,16 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts *opts, QCOW2_OPT_L2_CACHE_ENTRY_SIZE, s->cluster_size); + *l2_cache_size = MIN(max_l2_cache, l2_cache_max_setting); + if (combined_cache_size_set) { if (l2_cache_size_set && refcount_cache_size_set) { error_setg(errp, QCOW2_OPT_CACHE_SIZE ", " QCOW2_OPT_L2_CACHE_SIZE " and " QCOW2_OPT_REFCOUNT_CACHE_SIZE " may not be set " "at the same time"); return; - } else if (*l2_cache_size > combined_cache_size) { + } else if (l2_cache_size_set && + (l2_cache_max_setting > combined_cache_size)) { error_setg(errp, QCOW2_OPT_L2_CACHE_SIZE " may not exceed " QCOW2_OPT_CACHE_SIZE); return; @@ -815,13 +821,9 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts *opts, } else if (refcount_cache_size_set) { *l2_cache_size = combined_cache_size - *refcount_cache_size; } else { - uint64_t virtual_disk_size = bs->total_sectors * BDRV_SECTOR_SIZE; - uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8); - /* Assign as much memory as possible to the L2 cache, and * use the remainder for the refcount cache */ - if (combined_cache_size >= max_l2_cache + min_refcount_cache) { - *l2_cache_size = max_l2_cache; + if (combined_cache_size >= *l2_cache_size + min_refcount_cache) { *refcount_cache_size = combined_cache_size - *l2_cache_size; } else { *refcount_cache_size = MIN(combined_cache_size, @@ -829,10 +831,6 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts *opts, *l2_cache_size = combined_cache_size - *refcount_cache_size; } } - } else if (!l2_cache_size_set) { - *l2_cache_size = MAX(DEFAULT_L2_CACHE_SIZE, - (uint64_t)DEFAULT_L2_CACHE_CLUSTERS - * s->cluster_size); } /* If refcount-cache-size is not specified, it will be set to minimum * in qcow2_update_options_prepare(). No need to set it here. */ diff --git a/block/qcow2.h b/block/qcow2.h index 39e1b279f8..d917b5f577 100644 --- a/block/qcow2.h +++ b/block/qcow2.h @@ -74,9 +74,7 @@ /* Must be at least 4 to cover all cases of refcount table growth */ #define MIN_REFCOUNT_CACHE_SIZE 4 /* clusters */ -/* Whichever is more */ -#define DEFAULT_L2_CACHE_CLUSTERS 8 /* clusters */ -#define DEFAULT_L2_CACHE_SIZE (1 * MiB) +#define DEFAULT_L2_CACHE_MAX_SIZE (1 * MiB) #define DEFAULT_CLUSTER_SIZE (64 * KiB) diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt index 0f157d859a..69af306267 100644 --- a/docs/qcow2-cache.txt +++ b/docs/qcow2-cache.txt @@ -124,8 +124,11 @@ 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 default L2 cache size will cover the entire virtual size of an + image, up to a certain maximum. This maximum is 1 MB by default + (enough for image sizes of up to 8 GB with the default cluster size) + and it can be reduced or enlarged using the "l2-cache-size" option. + The minimum is 2 clusters (or 2 cache entries, see below). - The default (and minimum) refcount cache size is 4 clusters. @@ -183,9 +186,9 @@ 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. + (the default behavior for images of up to 8 GB in size) then none + of this is necessary and you can omit the "l2-cache-entry-size" + parameter altogether. Reducing the memory usage diff --git a/qemu-options.hx b/qemu-options.hx index f6804758d3..22e8e2d113 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -756,9 +756,9 @@ The maximum total size of the L2 table and refcount block caches in bytes @item l2-cache-size The maximum size of the L2 table cache in bytes -(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) +(default: if cache-size is not specified - 1M; otherwise, as large as possible +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 diff --git a/tests/qemu-iotests/137 b/tests/qemu-iotests/137 index 87965625d8..e3fb078588 100755 --- a/tests/qemu-iotests/137 +++ b/tests/qemu-iotests/137 @@ -109,7 +109,6 @@ $QEMU_IO \ -c "reopen -o cache-size=1M,l2-cache-size=64k,refcount-cache-size=64k" \ -c "reopen -o cache-size=1M,l2-cache-size=2M" \ -c "reopen -o cache-size=1M,refcount-cache-size=2M" \ - -c "reopen -o l2-cache-size=256T" \ -c "reopen -o l2-cache-entry-size=33k" \ -c "reopen -o l2-cache-entry-size=128k" \ -c "reopen -o refcount-cache-size=256T" \ diff --git a/tests/qemu-iotests/137.out b/tests/qemu-iotests/137.out index 6a2ffc71fd..70f245ae7a 100644 --- a/tests/qemu-iotests/137.out +++ b/tests/qemu-iotests/137.out @@ -19,7 +19,6 @@ Parameter 'lazy-refcounts' expects 'on' or 'off' cache-size, l2-cache-size and refcount-cache-size may not be set at the same time l2-cache-size may not exceed cache-size refcount-cache-size may not exceed cache-size -L2 cache size too big L2 cache entry size must be a power of two between 512 and the cluster size (65536) L2 cache entry size must be a power of two between 512 and the cluster size (65536) Refcount cache size too big -- 2.17.1