[Qemu-devel] [PATCH 24/27] block/parallels: delay writing to BAT till bdrv_co_flush_to_os

[Denis V. Lunev]

The idea is that we do not need to immediately sync BAT to the image as
from the guest point of view there is a possibility that IO is lost
even in the physical controller until flush command was finished.
bdrv_co_flush_to_os is exactly the right place for this purpose.

Technically the patch uses loaded BAT data as a cache and performs
actual on-disk metadata updates in parallels_co_flush_to_os callback.

This patch speed ups
  qemu-img create -f parallels -o cluster_size=64k ./1.hds 64G
  qemu-io -f parallels -c "write -P 0x11 0 1024k" 1.hds
writing from 50-60 Mb/sec to 80-90 Mb/sec on rotational media and
from 160 Mb/sec to 190 Mb/sec on SSD disk.

Signed-off-by: Denis V. Lunev <d...@openvz.org>
Reviewed-by: Roman Kagan <rka...@parallels.com>
CC: Kevin Wolf <kw...@redhat.com>
CC: Stefan Hajnoczi <stefa...@redhat.com>
---
 block/parallels.c | 50 ++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 44 insertions(+), 6 deletions(-)

diff --git a/block/parallels.c b/block/parallels.c
index f0518be..6e7f000 100644
--- a/block/parallels.c
+++ b/block/parallels.c
@@ -30,6 +30,7 @@
 #include "qemu-common.h"
 #include "block/block_int.h"
 #include "qemu/module.h"
+#include "qemu/bitmap.h"
 
 /**************************************************************/
 
@@ -62,6 +63,9 @@ typedef struct BDRVParallelsState {
     uint32_t header_size;
     bool header_unclean;
 
+    unsigned long *bat_dirty_bmap;
+    unsigned int  bat_dirty_block;
+
     uint32_t *bat_bitmap;
     unsigned int bat_size;
 
@@ -136,7 +140,6 @@ static int64_t allocate_cluster(BlockDriverState *bs, 
int64_t sector_num)
     BDRVParallelsState *s = bs->opaque;
     uint32_t idx, offset;
     int64_t pos;
-    int ret;
 
     idx = sector_num / s->tracks;
     offset = sector_num % s->tracks;
@@ -152,14 +155,43 @@ static int64_t allocate_cluster(BlockDriverState *bs, 
int64_t sector_num)
     bdrv_truncate(bs->file, (pos + s->tracks) << BDRV_SECTOR_BITS);
 
     s->bat_bitmap[idx] = cpu_to_le32(pos / s->off_multiplier);
-    ret = bdrv_pwrite(bs->file, bat_entry_off(idx), s->bat_bitmap + idx,
-                      sizeof(s->bat_bitmap[idx]));
-    if (ret < 0) {
-        return ret;
-    }
+
+    bitmap_set(s->bat_dirty_bmap, bat_entry_off(idx) / s->bat_dirty_block, 1);
     return bat2sect(s, idx) + offset;
 }
 
+
+static coroutine_fn int parallels_co_flush_to_os(BlockDriverState *bs)
+{
+    BDRVParallelsState *s = bs->opaque;
+    unsigned long size = DIV_ROUND_UP(s->header_size, s->bat_dirty_block);
+    unsigned long bit;
+
+    qemu_co_mutex_lock(&s->lock);
+
+    bit = find_first_bit(s->bat_dirty_bmap, size);
+    while (bit < size) {
+        uint32_t off = bit * s->bat_dirty_block;
+        uint32_t to_write = s->bat_dirty_block;
+        int ret;
+
+        if (off + to_write > s->header_size) {
+            to_write = s->header_size - off;
+        }
+        ret = bdrv_pwrite(bs->file, off, (uint8_t *)s->header + off, to_write);
+        if (ret < 0) {
+            qemu_co_mutex_unlock(&s->lock);
+            return ret;
+        }
+        bit = find_next_bit(s->bat_dirty_bmap, size, bit + 1);
+    }
+    bitmap_zero(s->bat_dirty_bmap, size);
+
+    qemu_co_mutex_unlock(&s->lock);
+    return 0;
+}
+
+
 static int64_t coroutine_fn parallels_co_get_block_status(BlockDriverState *bs,
         int64_t sector_num, int nb_sectors, int *pnum)
 {
@@ -542,6 +574,10 @@ static int parallels_open(BlockDriverState *bs, QDict 
*options, int flags,
         }
     }
 
+    s->bat_dirty_block = 4 * getpagesize();
+    s->bat_dirty_bmap =
+        bitmap_new(DIV_ROUND_UP(s->header_size, s->bat_dirty_block));
+
     qemu_co_mutex_init(&s->lock);
     return 0;
 
@@ -563,6 +599,7 @@ static void parallels_close(BlockDriverState *bs)
         parallels_update_header(bs);
     }
 
+    g_free(s->bat_dirty_bmap);
     qemu_vfree(s->header);
 }
 
@@ -592,6 +629,7 @@ static BlockDriver bdrv_parallels = {
     .bdrv_open         = parallels_open,
     .bdrv_close                = parallels_close,
     .bdrv_co_get_block_status = parallels_co_get_block_status,
+    .bdrv_co_flush_to_os      = parallels_co_flush_to_os,
     .bdrv_co_readv  = parallels_co_readv,
     .bdrv_co_writev = parallels_co_writev,
 
-- 
1.9.1