[Qemu-devel] [PATCH 1/1] mirror: double performance of the bulk stage if the disc is full

["Denis V. Lunev" <den@openvz.org>]

From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

Mirror can do up to 16 in-flight requests, but actually on full copy
(the whole source disk is non-zero) in-flight is always 1. This happens
as the request is not limited in size: the data occupies maximum available
capacity of s->buf.

The patch limits the size of the request to some artificial constant
(1 Mb here), which is not that big or small. This effectively enables
back parallelism in mirror code as it was designed.

The result is important: the time to migrate 10 Gb disk is reduced from
~350 sec to 170 sec.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <famz@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Jeff Cody <jcody@redhat.com>
CC: Eric Blake <eblake@redhat.com>
---
 block/mirror.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/block/mirror.c b/block/mirror.c
index 4fe127e..53d3bcd 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -23,7 +23,9 @@
 
 #define SLICE_TIME    100000000ULL /* ns */
 #define MAX_IN_FLIGHT 16
-#define DEFAULT_MIRROR_BUF_SIZE   (10 << 20)
+#define MAX_IO_SECTORS ((1 << 20) >> BDRV_SECTOR_BITS) /* 1 Mb */
+#define DEFAULT_MIRROR_BUF_SIZE \
+    (MAX_IN_FLIGHT * MAX_IO_SECTORS * BDRV_SECTOR_SIZE)
 
 /* The mirroring buffer is a list of granularity-sized chunks.
  * Free chunks are organized in a list.
@@ -387,7 +389,9 @@ static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
                                           nb_chunks * sectors_per_chunk,
                                           &io_sectors, &file);
         if (ret < 0) {
-            io_sectors = nb_chunks * sectors_per_chunk;
+            io_sectors = MIN(nb_chunks * sectors_per_chunk, MAX_IO_SECTORS);
+        } else if (ret & BDRV_BLOCK_DATA) {
+            io_sectors = MIN(io_sectors, MAX_IO_SECTORS);
         }
 
         io_sectors -= io_sectors % sectors_per_chunk;
-- 
2.1.4