qcow2: Constant cache size in bytes

Specifying the metadata cache sizes in clusters results in less clusters
(and much less bytes) covered for small cluster sizes and vice versa.
Using a constant byte size reduces this difference, and makes it
possible to manually specify the cache size in an easily comprehensible
unit.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2.c

@@ -470,6 +470,7 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
     uint64_t l1_vm_state_index;
     const char *opt_overlap_check;
     int overlap_check_template = 0;
+    uint64_t l2_cache_size, refcount_cache_size;
 
     ret = bdrv_pread(bs->file, 0, &header, sizeof(header));
     if (ret < 0) {
@@ -707,8 +708,18 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
     }
 
     /* alloc L2 table/refcount block cache */
-    s->l2_table_cache = qcow2_cache_create(bs, L2_CACHE_SIZE);
-    s->refcount_block_cache = qcow2_cache_create(bs, REFCOUNT_CACHE_SIZE);
+    l2_cache_size = DEFAULT_L2_CACHE_BYTE_SIZE / s->cluster_size;
+    if (l2_cache_size < MIN_L2_CACHE_SIZE) {
+        l2_cache_size = MIN_L2_CACHE_SIZE;
+    }
+
+    refcount_cache_size = l2_cache_size / DEFAULT_L2_REFCOUNT_SIZE_RATIO;
+    if (refcount_cache_size < MIN_REFCOUNT_CACHE_SIZE) {
+        refcount_cache_size = MIN_REFCOUNT_CACHE_SIZE;
+    }
+
+    s->l2_table_cache = qcow2_cache_create(bs, l2_cache_size);
+    s->refcount_block_cache = qcow2_cache_create(bs, refcount_cache_size);
     if (s->l2_table_cache == NULL || s->refcount_block_cache == NULL) {
         error_setg(errp, "Could not allocate metadata caches");
         ret = -ENOMEM;

block/qcow2.h

@@ -64,10 +64,16 @@
 #define MIN_CLUSTER_BITS 9
 #define MAX_CLUSTER_BITS 21
 
-#define L2_CACHE_SIZE 16
+#define MIN_L2_CACHE_SIZE 1 /* cluster */
 
 /* Must be at least 4 to cover all cases of refcount table growth */
-#define REFCOUNT_CACHE_SIZE 4
+#define MIN_REFCOUNT_CACHE_SIZE 4 /* clusters */
+
+#define DEFAULT_L2_CACHE_BYTE_SIZE 1048576 /* bytes */
+
+/* The refblock cache needs only a fourth of the L2 cache size to cover as many
+ * clusters */
+#define DEFAULT_L2_REFCOUNT_SIZE_RATIO 4
 
 #define DEFAULT_CLUSTER_SIZE 65536