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@xxxxxx>
> ---
> 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
>
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