[Qemu-devel] [PATCH v3] mirror: Only mirror granularity-aligned chunks

[Max Reitz <mreitz@redhat.com>]

In write-blocking mode, all writes to the top node directly go to the
target.  We must only mirror chunks of data that are aligned to the
job's granularity, because that is how the dirty bitmap works.
Therefore, the request alignment for writes must be the job's
granularity (in write-blocking mode).

Unfortunately, this forces all reads and writes to have the same
granularity (we only need this alignment for writes to the target, not
the source), but that is something to be fixed another time.

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
---
v3: Handle bdrv_refresh_limits() errors [Vladimir]
---
 block/mirror.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/block/mirror.c b/block/mirror.c
index 8cb75fb409..9f5c59ece1 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -1481,6 +1481,15 @@ static void bdrv_mirror_top_child_perm(BlockDriverState *bs, BdrvChild *c,
     *nshared = BLK_PERM_ALL;
 }
 
+static void bdrv_mirror_top_refresh_limits(BlockDriverState *bs, Error **errp)
+{
+    MirrorBDSOpaque *s = bs->opaque;
+
+    if (s && s->job && s->job->copy_mode == MIRROR_COPY_MODE_WRITE_BLOCKING) {
+        bs->bl.request_alignment = s->job->granularity;
+    }
+}
+
 /* Dummy node that provides consistent read to its users without requiring it
  * from its backing file and that allows writes on the backing file chain. */
 static BlockDriver bdrv_mirror_top = {
@@ -1493,6 +1502,7 @@ static BlockDriver bdrv_mirror_top = {
     .bdrv_co_block_status       = bdrv_co_block_status_from_backing,
     .bdrv_refresh_filename      = bdrv_mirror_top_refresh_filename,
     .bdrv_child_perm            = bdrv_mirror_top_child_perm,
+    .bdrv_refresh_limits        = bdrv_mirror_top_refresh_limits,
 };
 
 static BlockJob *mirror_start_job(
@@ -1637,6 +1647,25 @@ static BlockJob *mirror_start_job(
         s->should_complete = true;
     }
 
+    /*
+     * Must be called before we start tracking writes, but after
+     *
+     *     ((MirrorBlockJob *)
+     *         ((MirrorBDSOpaque *)
+     *             mirror_top_bs->opaque
+     *         )->job
+     *     )->copy_mode
+     *
+     * has the correct value.
+     * (We start tracking writes as of the following
+     * bdrv_create_dirty_bitmap() call.)
+     */
+    bdrv_refresh_limits(mirror_top_bs, &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
+        goto fail;
+    }
+
     s->dirty_bitmap = bdrv_create_dirty_bitmap(bs, granularity, NULL, errp);
     if (!s->dirty_bitmap) {
         goto fail;
-- 
2.21.0