Re: [PATCH 14/14] Btrfs: don't do nocow check unless we have to

From: Liu Bo <bo.li.liu@oracle.com>
To: Josef Bacik <jbacik@fb.com>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 14/14] Btrfs: don't do nocow check unless we have to
Date: Fri, 25 Mar 2016 10:50:00 -0700	[thread overview]
Message-ID: <20160325175000.GA22147@localhost.localdomain> (raw)
In-Reply-To: <1458926760-17563-15-git-send-email-jbacik@fb.com>

On Fri, Mar 25, 2016 at 01:26:00PM -0400, Josef Bacik wrote:
> Before we write into prealloc/nocow space we have to make sure that there are no
> references to the extents we are writing into, which means checking the extent
> tree and csum tree in the case of nocow.  So we don't want to do the nocow dance
> unless we can't reserve data space, since it's a serious drag on performance.
> With the following sequence
> 
> fallocate -l10737418240 /mnt/btrfs-test/file
> cp --reflink /mnt/btrfs-test/file /mnt/btrfs-test/link
> fio --name=randwrite --rw=randwrite --bs=4k --filename=/mnt/btrfs-test/file \
> 	--end_fsync=1
> 
> we get the worst case scenario where we have to fall back on to doing the check
> anyway.
> 
> Without this patch
> lat (usec): min=5, max=111598, avg=27.65, stdev=124.51
> write: io=10240MB, bw=126876KB/s, iops=31718, runt= 82646msec
> 
> With this patch
> lat (usec): min=3, max=91210, avg=14.09, stdev=110.62
> write: io=10240MB, bw=212753KB/s, iops=53188, runt= 49286msec
> 
> We get twice the throughput, half of the runtime, and half of the average
> latency.  Thanks,

I've submitted a similar one, but looks like this one is cleaner, I
forgot to remove the goto reserve_metadata.

Thanks,

-liubo

> 
> Signed-off-by: Josef Bacik <jbacik@fb.com>
> ---
>  fs/btrfs/file.c | 44 ++++++++++++++++++++++----------------------
>  1 file changed, 22 insertions(+), 22 deletions(-)
> 
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 0ce4bb3..7c80208 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -1534,30 +1534,30 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
>  		reserve_bytes = round_up(write_bytes + sector_offset,
>  				root->sectorsize);
>  
> -		if ((BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
> -					      BTRFS_INODE_PREALLOC)) &&
> -		    check_can_nocow(inode, pos, &write_bytes) > 0) {
> -			/*
> -			 * For nodata cow case, no need to reserve
> -			 * data space.
> -			 */
> -			only_release_metadata = true;
> -			/*
> -			 * our prealloc extent may be smaller than
> -			 * write_bytes, so scale down.
> -			 */
> -			num_pages = DIV_ROUND_UP(write_bytes + offset,
> -						 PAGE_CACHE_SIZE);
> -			reserve_bytes = round_up(write_bytes + sector_offset,
> -					root->sectorsize);
> -			goto reserve_metadata;
> -		}
> -
>  		ret = btrfs_check_data_free_space(inode, pos, write_bytes);
> -		if (ret < 0)
> -			break;
> +		if (ret < 0) {
> +			if ((BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
> +						      BTRFS_INODE_PREALLOC)) &&
> +			    check_can_nocow(inode, pos, &write_bytes) > 0) {
> +				/*
> +				 * For nodata cow case, no need to reserve
> +				 * data space.
> +				 */
> +				only_release_metadata = true;
> +				/*
> +				 * our prealloc extent may be smaller than
> +				 * write_bytes, so scale down.
> +				 */
> +				num_pages = DIV_ROUND_UP(write_bytes + offset,
> +							 PAGE_CACHE_SIZE);
> +				reserve_bytes = round_up(write_bytes +
> +							 sector_offset,
> +							 root->sectorsize);
> +			} else {
> +				break;
> +			}
> +		}
>  
> -reserve_metadata:
>  		ret = btrfs_delalloc_reserve_metadata(inode, reserve_bytes);
>  		if (ret) {
>  			if (!only_release_metadata)
> -- 
> 2.5.0
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html