[Qemu-devel] [PATCH 1/3] qcow2: Factor next_refcount_table_size out

[Kevin Wolf <kwolf@redhat.com>]

When the refcount table grows, it doesn't only grow by one entry but reserves
some space for future refcount blocks. The algorithm to calculate the number of
entries stays the same with the fixes, so factor it out before replacing the
rest.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2-refcount.c |   34 +++++++++++++++++++++++-----------
 1 files changed, 23 insertions(+), 11 deletions(-)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 2fdc26b..0e2ecd7 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -123,6 +123,28 @@ static int get_refcount(BlockDriverState *bs, int64_t cluster_index)
     return be16_to_cpu(s->refcount_block_cache[block_index]);
 }
 
+/*
+ * Rounds the refcount table size up to avoid growing the table for each single
+ * refcount block that is allocated.
+ */
+static unsigned int next_refcount_table_size(BDRVQcowState *s,
+    unsigned int min_size)
+{
+    unsigned int refcount_table_clusters = 0;
+    unsigned int new_table_size = 1;
+
+    while (min_size > new_table_size) {
+        if (refcount_table_clusters == 0) {
+            refcount_table_clusters = 1;
+        } else {
+            refcount_table_clusters = (refcount_table_clusters * 3 + 1) / 2;
+        }
+        new_table_size = refcount_table_clusters << (s->cluster_bits - 3);
+    }
+
+    return new_table_size;
+}
+
 static int grow_refcount_table(BlockDriverState *bs, int min_size)
 {
     BDRVQcowState *s = bs->opaque;
@@ -136,17 +158,7 @@ static int grow_refcount_table(BlockDriverState *bs, int min_size)
     if (min_size <= s->refcount_table_size)
         return 0;
     /* compute new table size */
-    refcount_table_clusters = s->refcount_table_size >> (s->cluster_bits - 3);
-    for(;;) {
-        if (refcount_table_clusters == 0) {
-            refcount_table_clusters = 1;
-        } else {
-            refcount_table_clusters = (refcount_table_clusters * 3 + 1) / 2;
-        }
-        new_table_size = refcount_table_clusters << (s->cluster_bits - 3);
-        if (min_size <= new_table_size)
-            break;
-    }
+    new_table_size = next_refcount_table_size(s, min_size);
 #ifdef DEBUG_ALLOC2
     printf("grow_refcount_table from %d to %d\n",
            s->refcount_table_size,
-- 
1.6.6