qcow2: Default to 4KB for the qcow2 cache entry size

QEMU 2.12 (commit 1221fe6f636754ab5f2c1c87caa77633e9123622) introduced
a new setting called l2-cache-entry-size that allows making entries on
the qcow2 L2 cache smaller than the cluster size.

I have been performing several tests with different cluster and entry
sizes and all of them show that reducing the entry size (aka L2 slice)
consistently improves I/O performance, notably during random I/O (all
tests done with sequential I/O show similar results). This is to be
expected because loading and evicting an L2 slice is more expensive
the larger the slice is.

Here are some numbers on fully populated 40GB qcow2 images. The
rightmost column represents the maximum L2 cache size in both cases.

   Cluster size = 64 KB
   |-------------+--------------+--------------+--------------|
   |             | 1MB L2 cache | 3MB L2 cache | 5MB L2 cache |
   |-------------+--------------+--------------+--------------|
   |  4KB slices |    6545 IOPS |   12045 IOPS |   55680 IOPS |
   | 16KB slices |    5177 IOPS |    9798 IOPS |   56278 IOPS |
   | 64KB slices |    2718 IOPS |    5326 IOPS |   57355 IOPS |
   |-------------+--------------+--------------+--------------|

   Cluster size = 256 KB
   |--------------+----------------+--------------+-----------------|
   |              | 512KB L2 cache | 1MB L2 cache | 1280KB L2 cache |
   |--------------+----------------+--------------+-----------------|
   |   4KB slices |      8539 IOPS |   21071 IOPS |      55417 IOPS |
   |  64KB slices |      3598 IOPS |    9772 IOPS |      57687 IOPS |
   | 256KB slices |      1415 IOPS |    4120 IOPS |      58001 IOPS |
   |--------------+----------------+--------------+-----------------|

As can be seen in the numbers, the only exception to the rule is when
the cache is large enough to hold all L2 tables. This is also to be
expected because in this case no cache entry is ever evicted so
reducing its size doesn't bring any benefit.

This patch sets the default L2 cache entry size to 4KB except when the
cache is large enough for the whole disk.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/block/qcow2.c b/block/qcow2.c
index 7fb2730..48d22f4 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -788,6 +788,7 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts *opts,
     BDRVQcow2State *s = bs->opaque;
     uint64_t combined_cache_size, l2_cache_max_setting;
     bool l2_cache_size_set, refcount_cache_size_set, combined_cache_size_set;
+    bool l2_cache_entry_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);
@@ -795,6 +796,7 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts *opts,
     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);
+    l2_cache_entry_size_set = qemu_opt_get(opts, QCOW2_OPT_L2_CACHE_ENTRY_SIZE);
 
     combined_cache_size = qemu_opt_get_size(opts, QCOW2_OPT_CACHE_SIZE, 0);
     l2_cache_max_setting = qemu_opt_get_size(opts, QCOW2_OPT_L2_CACHE_SIZE,
@@ -841,6 +843,16 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts *opts,
             }
         }
     }
+
+    /*
+     * If the L2 cache is not enough to cover the whole disk then
+     * default to 4KB entries. Smaller entries reduce the cost of
+     * loads and evictions and increase I/O performance.
+     */
+    if (*l2_cache_size < max_l2_cache && !l2_cache_entry_size_set) {
+        *l2_cache_entry_size = MIN(s->cluster_size, 4096);
+    }
+
     /* l2_cache_size and refcount_cache_size are ensured to have at least
      * their minimum values in qcow2_update_options_prepare() */