Re: [Qemu-devel] [PATCH 21/27] block/parallels: no need to flush on each block allocation table update

[Stefan Hajnoczi]

On Wed, Mar 11, 2015 at 01:28:15PM +0300, Denis V. Lunev wrote:
> From the point of guest each write to real disk prior to disk barrier
> operation could be lost. Therefore there is no problem that "not synced"
> new block is lost due to not updated allocation table if QEMU is crashed.
> This situation is properly detected and handled now using inuse magic
> and in parallels_check
> 
> This patch improves writing performance of
>   qemu-img create -f parallels -o cluster_size=64k ./1.hds 64G
>   qemu-io -f parallels -c "write -P 0x11 0 1024k" 1.hds
> from 45 Mb/sec to 160 Mb/sec on my SSD disk. The gain on rotational media
> is much more sufficient, from 800 Kb/sec to 45 Mb/sec.
> 
> Signed-off-by: Denis V. Lunev <address@hidden>
> Reviewed-by: Roman Kagan <address@hidden>
> CC: Kevin Wolf <address@hidden>
> CC: Stefan Hajnoczi <address@hidden>
> ---
>  block/parallels.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/parallels.c b/block/parallels.c
> index bafc74b..2605c1a 100644
> --- a/block/parallels.c
> +++ b/block/parallels.c
> @@ -118,7 +118,7 @@ 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_sync(bs->file,
> +    ret = bdrv_pwrite(bs->file,
>              sizeof(ParallelsHeader) + idx * sizeof(s->bat_bitmap[idx]),
>              s->bat_bitmap + idx, sizeof(s->bat_bitmap[idx]));
>      if (ret < 0) {

Please squash this into the write support patch.

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