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[Qemu-block] [PATCH] mirror: Only mirror granularity-aligned chunks

From: Max Reitz
Subject: [Qemu-block] [PATCH] mirror: Only mirror granularity-aligned chunks
Date: Mon, 5 Aug 2019 13:49:23 +0200

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.

Signed-off-by: Max Reitz <address@hidden>
This is an alternative to Vladimir's "util/hbitmap: fix unaligned reset"
patch.  I don't mind much either way, both of pros and cons.  Comparing
this patch to Vladimir's:

+ Makes copy-mode=write-blocking really work (unless I'm mistaken)
- Lowers performance with copy-mode=write-blocking unnecessarily
 block/mirror.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/block/mirror.c b/block/mirror.c
index 8cb75fb409..3f9c5a178a 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(
@@ -1678,6 +1688,8 @@ static BlockJob *mirror_start_job(
+    bdrv_refresh_limits(mirror_top_bs, &error_abort);
     trace_mirror_start(bs, s, opaque);

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