[Qemu-devel] [PATCH v2 2/5] qcow2: Make the default L2 cache sufficient to cover the entire image

[Leonid Bloch]

Sufficient L2 cache can noticeably improve the performance when using
large images with frequent I/O. The memory overhead is not significant
in most cases, as the cache size is only 1 MB for each 8 GB of virtual
image size (with the default cluster size of 64 KB). For cases with very
large images and/or small cluster sizes, there is an upper limit on the
default L2 cache: 32 MB. To modify this limit one can use the already
existing '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 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 <address@hidden>
---
 block/qcow2.c              | 33 +++++++++++++--------------------
 block/qcow2.h              |  4 +---
 docs/qcow2-cache.txt       | 24 ++++++++++++++----------
 qemu-options.hx            |  6 +++---
 tests/qemu-iotests/137     |  1 -
 tests/qemu-iotests/137.out |  1 -
 6 files changed, 31 insertions(+), 38 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index ec9e6238a0..98cb96aaca 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -777,29 +777,35 @@ 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 min_refcount_cache = MIN_REFCOUNT_CACHE_SIZE * s->cluster_size;
 
     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);
 
     *l2_cache_entry_size = qemu_opt_get_size(
         opts, QCOW2_OPT_L2_CACHE_ENTRY_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);
+    *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;
@@ -814,29 +820,16 @@ 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, min_refcount_cache);
+                *refcount_cache_size = MIN(combined_cache_size,
+                                           min_refcount_cache);
                 *l2_cache_size = combined_cache_size - *refcount_cache_size;
             }
         }
-    } else {
-        if (!l2_cache_size_set) {
-            *l2_cache_size = MAX(DEFAULT_L2_CACHE_BYTE_SIZE,
-                                 (uint64_t)DEFAULT_L2_CACHE_CLUSTERS
-                                 * s->cluster_size);
-        }
-        if (!refcount_cache_size_set) {
-            *refcount_cache_size = min_refcount_cache;
-        }
     }
 
     if (*l2_cache_entry_size < (1 << MIN_CLUSTER_BITS) ||
diff --git a/block/qcow2.h b/block/qcow2.h
index 81b844e936..d77a31d932 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -73,9 +73,7 @@
 /* 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_BYTE_SIZE 1048576 /* bytes */
+#define DEFAULT_L2_CACHE_MAX_SIZE 0x2000000U /* bytes */
 
 #define DEFAULT_CLUSTER_SIZE 65536
 
diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
index 5bf2a8ad29..c7625cdeb3 100644
--- a/docs/qcow2-cache.txt
+++ b/docs/qcow2-cache.txt
@@ -97,12 +97,14 @@ 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
+QEMU will use a default L2 cache sufficient to cover the entire virtual
+size of an image, which with the default cluster size will result in 1 MB
+of cache for every 8 GB of virtual image size:
 
-   1048576 / 131072 = 8 GB of virtual disk covered by that cache
-    262144 /  32768 = 8 GB
+   65536 / 8 = 8192 = 8 GB / 1 MB
+
+A default refcount cache is 4 times the cluster size, which defaults to
+256 KB (262144 bytes).
 
 
 How to configure the cache sizes
@@ -121,8 +123,11 @@ 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, but is capped at 32 MB (enough for image sizes of up to 256 GB
+   with the default cluster size). This maximum value 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.
 
@@ -180,9 +185,8 @@ 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) 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..d6e15b2f06 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 defined - 32M; 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
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