[Qemu-devel] [PATCH] qcow2: Cache refcount blocks during snapshot creation

[Kevin Wolf <kwolf@redhat.com>]

The really time consuming part of snapshotting is to adjust the reference count
of all clusters. Currently after each adjusted cluster the refcount block is
written to disk.

Don't write each single byte immediately to disk but cache all writes to the
refcount block and write them out once we're done with the block.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2-refcount.c |   40 ++++++++++++++++++++++++++++++++++++++++
 1 files changed, 40 insertions(+), 0 deletions(-)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index dd6e293..d42c6e6 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -31,6 +31,26 @@ static int update_refcount(BlockDriverState *bs,
                             int64_t offset, int64_t length,
                             int addend);
 
+
+static int cache_refcount_updates = 0;
+
+static int write_refcount_block(BDRVQcowState *s)
+{
+    size_t size = s->cluster_size;
+
+    if (s->refcount_block_cache_offset == 0) {
+        return 0;
+    }
+
+    if (bdrv_pwrite(s->hd, s->refcount_block_cache_offset,
+            s->refcount_block_cache, size) != size)
+    {
+        return -EIO;
+    }
+
+    return 0;
+}
+
 /*********************************************************/
 /* refcount handling */
 
@@ -68,6 +88,11 @@ static int load_refcount_block(BlockDriverState *bs,
 {
     BDRVQcowState *s = bs->opaque;
     int ret;
+
+    if (cache_refcount_updates) {
+        write_refcount_block(s);
+    }
+
     ret = bdrv_pread(s->hd, refcount_block_offset, s->refcount_block_cache,
                      s->cluster_size);
     if (ret != s->cluster_size)
@@ -169,6 +194,7 @@ static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
     int64_t offset, refcount_block_offset;
     int ret, refcount_table_index;
     uint64_t data64;
+    int cache = cache_refcount_updates;
 
     /* Find L1 index and grow refcount table if needed */
     refcount_table_index = cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);
@@ -181,6 +207,10 @@ static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
     /* Load or allocate the refcount block */
     refcount_block_offset = s->refcount_table[refcount_table_index];
     if (!refcount_block_offset) {
+        if (cache_refcount_updates) {
+            write_refcount_block(s);
+            cache_refcount_updates = 0;
+        }
         /* create a new refcount block */
         /* Note: we cannot update the refcount now to avoid recursion */
         offset = alloc_clusters_noref(bs, s->cluster_size);
@@ -199,6 +229,7 @@ static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
         refcount_block_offset = offset;
         s->refcount_block_cache_offset = offset;
         update_refcount(bs, offset, s->cluster_size, 1);
+        cache_refcount_updates = cache;
     } else {
         if (refcount_block_offset != s->refcount_block_cache_offset) {
             if (load_refcount_block(bs, refcount_block_offset) < 0)
@@ -215,6 +246,10 @@ static int write_refcount_block_entries(BDRVQcowState *s,
 {
     size_t size;
 
+    if (cache_refcount_updates) {
+        return 0;
+    }
+
     first_index &= ~(REFCOUNTS_PER_SECTOR - 1);
     last_index = (last_index + REFCOUNTS_PER_SECTOR)
         & ~(REFCOUNTS_PER_SECTOR - 1);
@@ -471,6 +506,7 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
     int l2_size, i, j, l1_modified, l2_modified, nb_csectors, refcount;
 
     qcow2_l2_cache_reset(bs);
+    cache_refcount_updates = 1;
 
     l2_table = NULL;
     l1_table = NULL;
@@ -563,11 +599,15 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
     if (l1_allocated)
         qemu_free(l1_table);
     qemu_free(l2_table);
+    cache_refcount_updates = 0;
+    write_refcount_block(s);
     return 0;
  fail:
     if (l1_allocated)
         qemu_free(l1_table);
     qemu_free(l2_table);
+    cache_refcount_updates = 0;
+    write_refcount_block(s);
     return -EIO;
 }
 
-- 
1.6.0.6