[Qemu-devel] [PATCH] block: Change bdrv_commit to handle multiple sectors at once

[Kevin Wolf]

bdrv_commit copies the image to its backing file sector by sector, which
is (surprise!) relatively slow. Let's take a larger buffer and handle more
sectors at once if possible.

With a 1G qcow2 file, this brought the time bdrv_commit takes down from
5:06 min to 1:14 min for me.

Signed-off-by: Kevin Wolf <address@hidden>
---
 block.c |   35 ++++++++++++++++++-----------------
 1 files changed, 18 insertions(+), 17 deletions(-)

diff --git a/block.c b/block.c
index 65cf4dc..12dbc43 100644
--- a/block.c
+++ b/block.c
@@ -729,9 +729,9 @@ int bdrv_commit(BlockDriverState *bs)
 {
     BlockDriver *drv = bs->drv;
     int64_t i, total_sectors;
-    int n, j, ro, open_flags;
+    int n, ro, open_flags;
     int ret = 0, rw_ret = 0;
-    unsigned char sector[BDRV_SECTOR_SIZE];
+    uint8_t *buf;
     char filename[1024];
     BlockDriverState *bs_rw, *bs_ro;
 
@@ -774,25 +774,26 @@ int bdrv_commit(BlockDriverState *bs)
     }
 
     total_sectors = bdrv_getlength(bs) >> BDRV_SECTOR_BITS;
+    buf = qemu_malloc(2048 * BDRV_SECTOR_SIZE);
+
     for (i = 0; i < total_sectors;) {
-        if (drv->bdrv_is_allocated(bs, i, 65536, &n)) {
-            for(j = 0; j < n; j++) {
-                if (bdrv_read(bs, i, sector, 1) != 0) {
-                    ret = -EIO;
-                    goto ro_cleanup;
-                }
+        if (drv->bdrv_is_allocated(bs, i, 2048, &n)) {
 
-                if (bdrv_write(bs->backing_hd, i, sector, 1) != 0) {
-                    ret = -EIO;
-                    goto ro_cleanup;
-                }
-                i++;
-           }
-       } else {
-            i += n;
+            if (bdrv_read(bs, i, buf, n) != 0) {
+                ret = -EIO;
+                goto ro_cleanup;
+            }
+
+            if (bdrv_write(bs->backing_hd, i, buf, n) != 0) {
+                ret = -EIO;
+                goto ro_cleanup;
+            }
         }
+        i += n;
     }
 
+    qemu_free(buf);
+
     if (drv->bdrv_make_empty) {
         ret = drv->bdrv_make_empty(bs);
         bdrv_flush(bs);
-- 
1.7.1.1