From: Kevin Wolf <kwolf@redhat.com>
Date: Fri,  9 Apr 2010 11:46:21 +0200
Message-Id: <1270806388-28138-4-git-send-email-kwolf@redhat.com>
In-Reply-To: <1270806388-28138-1-git-send-email-kwolf@redhat.com>
References: <1270806388-28138-1-git-send-email-kwolf@redhat.com>
Subject: [Qemu-devel] [STABLE][PATCH 03/10] qcow2: Factor
	next_refcount_table_size out
To: aurelien@aurel32.net
Cc: kwolf@redhat.com, qemu-devel@nongnu.org

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.

As Juan suggested take the opportunity to simplify the code a bit.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit 05121aedc41f87e44e41e9cef55f2e49ce7ba94e)
---
 block/qcow2-refcount.c |   30 +++++++++++++++++++-----------
 1 files changed, 19 insertions(+), 11 deletions(-)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index c2a5c04..5dde80a 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -123,6 +123,24 @@ 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 min_clusters = (min_size >> (s->cluster_bits - 3)) + 1;
+    unsigned int refcount_table_clusters =
+        MAX(1, s->refcount_table_size >> (s->cluster_bits - 3));
+
+    while (min_clusters > refcount_table_clusters) {
+        refcount_table_clusters = (refcount_table_clusters * 3 + 1) / 2;
+    }
+
+    return refcount_table_clusters << (s->cluster_bits - 3);
+}
+
 static int grow_refcount_table(BlockDriverState *bs, int min_size)
 {
     BDRVQcowState *s = bs->opaque;
@@ -136,17 +154,7 @@ 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,
-- 
1.6.6.1