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[Qemu-block] [PATCH v2] migration/block: Avoid invoking blk_drain too fr

From: Lidong Chen
Subject: [Qemu-block] [PATCH v2] migration/block: Avoid invoking blk_drain too frequently
Date: Wed, 15 Mar 2017 11:37:33 +0800

Increase bmds->cur_dirty after submit io, so reduce the frequency
involve into blk_drain, and improve the performance obviously
when block migration.

The performance test result of this patch:

During the block dirty save phase, this patch improve guest os IOPS
from 4.0K to 9.5K. and improve the migration speed from
505856 rsec/s to 855756 rsec/s.

Signed-off-by: Lidong Chen <address@hidden>
 migration/block.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/migration/block.c b/migration/block.c
index 6741228..7734ff7 100644
--- a/migration/block.c
+++ b/migration/block.c
@@ -576,6 +576,9 @@ static int mig_save_device_dirty(QEMUFile *f, 
BlkMigDevState *bmds,
             bdrv_reset_dirty_bitmap(bmds->dirty_bitmap, sector, nr_sectors);
+            sector += nr_sectors;
+            bmds->cur_dirty = sector;
         sector += BDRV_SECTORS_PER_DIRTY_CHUNK;

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