Patchwork [v2,1/3] qcow2: Factor next_refcount_table_size out

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Submitter Kevin Wolf
Date Feb. 23, 2010, 3:40 p.m.
Message ID <1266939654-26510-2-git-send-email-kwolf@redhat.com>
Download mbox | patch
Permalink /patch/46069/
State New
Headers show

Comments

Kevin Wolf - Feb. 23, 2010, 3:40 p.m.
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>
---
 block/qcow2-refcount.c |   30 +++++++++++++++++++-----------
 1 files changed, 19 insertions(+), 11 deletions(-)
Juan Quintela - Feb. 23, 2010, 4 p.m.
Kevin Wolf <kwolf@redhat.com> wrote:
> 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>

Acked-by: Juan Quintela <quintela@redhat.com>

Patch

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 2fdc26b..933311c 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,