[Qemu-block] [PATCH v2 7/7] mirror: improve performance of mirroring of empty disk

[Denis V. Lunev]

We should not take into account zero blocks for delay calculations.
They are not read and thus IO throttling is not required. In the
other case VM migration with 16 Tb QCOW2 disk with 4 Gb of data takes
days.

Signed-off-by: Denis V. Lunev <address@hidden>
Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>
CC: Stefan Hajnoczi <address@hidden>
CC: Fam Zheng <address@hidden>
CC: Kevin Wolf <address@hidden>
CC: Max Reitz <address@hidden>
CC: Jeff Cody <address@hidden>
CC: Eric Blake <address@hidden>
---
 block/mirror.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/block/mirror.c b/block/mirror.c
index 4ecfbf1..3167de3 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -322,6 +322,7 @@ static uint64_t coroutine_fn 
mirror_iteration(MirrorBlockJob *s)
     int nb_chunks = 1;
     int64_t end = s->bdev_length / BDRV_SECTOR_SIZE;
     int sectors_per_chunk = s->granularity >> BDRV_SECTOR_BITS;
+    bool write_zeroes_ok = bdrv_can_write_zeroes_with_unmap(blk_bs(s->target));
 
     sector_num = hbitmap_iter_next(&s->hbi);
     if (sector_num < 0) {
@@ -372,7 +373,7 @@ static uint64_t coroutine_fn 
mirror_iteration(MirrorBlockJob *s)
     bitmap_set(s->in_flight_bitmap, sector_num / sectors_per_chunk, nb_chunks);
     while (nb_chunks > 0 && sector_num < end) {
         int ret;
-        int io_sectors;
+        int io_sectors, io_sectors_acct;
         BlockDriverState *file;
         enum MirrorMethod {
             MIRROR_METHOD_COPY,
@@ -409,12 +410,17 @@ static uint64_t coroutine_fn 
mirror_iteration(MirrorBlockJob *s)
         switch (mirror_method) {
         case MIRROR_METHOD_COPY:
             io_sectors = mirror_do_read(s, sector_num, io_sectors);
+            io_sectors_acct = io_sectors;
             break;
         case MIRROR_METHOD_ZERO:
-            mirror_do_zero_or_discard(s, sector_num, io_sectors, false);
-            break;
         case MIRROR_METHOD_DISCARD:
-            mirror_do_zero_or_discard(s, sector_num, io_sectors, true);
+            mirror_do_zero_or_discard(s, sector_num, io_sectors,
+                                      mirror_method == MIRROR_METHOD_DISCARD);
+            if (write_zeroes_ok) {
+                io_sectors_acct = 0;
+            } else {
+                io_sectors_acct = io_sectors;
+            }
             break;
         default:
             abort();
@@ -422,7 +428,7 @@ static uint64_t coroutine_fn 
mirror_iteration(MirrorBlockJob *s)
         assert(io_sectors);
         sector_num += io_sectors;
         nb_chunks -= DIV_ROUND_UP(io_sectors, sectors_per_chunk);
-        delay_ns += ratelimit_calculate_delay(&s->limit, io_sectors);
+        delay_ns += ratelimit_calculate_delay(&s->limit, io_sectors_acct);
     }
     return delay_ns;
 }
-- 
2.5.0