On 1.11.18 г. 8:49 ч., Ethan Lien wrote:
> Snapshot is expected to be fast. But if there are writers steadily
> create dirty pages in our subvolume, the snapshot may take a very long
> time to complete. To fix the problem, we use tagged writepage for snapshot
> flusher as we do in the generic write_cache_pages(), so we can ommit pages
> dirtied after the snapshot command.
>
> We do a simple snapshot speed test on a Intel D-1531 box:
>
> fio --ioengine=libaio --iodepth=32 --bs=4k --rw=write --size=64G
> --direct=0 --thread=1 --numjobs=1 --time_based --runtime=120
> --filename=/mnt/sub/testfile --name=job1 --group_reporting & sleep 5;
> time btrfs sub snap -r /mnt/sub /mnt/snap; killall fio
>
> original: 1m58sec
> patched: 6.54sec
>
> This is the best case for this patch since for a sequential write case,
> we omit nearly all pages dirtied after the snapshot command.
>
> For a multi writers, random write test:
>
> fio --ioengine=libaio --iodepth=32 --bs=4k --rw=randwrite --size=64G
> --direct=0 --thread=1 --numjobs=4 --time_based --runtime=120
> --filename=/mnt/sub/testfile --name=job1 --group_reporting & sleep 5;
> time btrfs sub snap -r /mnt/sub /mnt/snap; killall fio
>
> original: 15.83sec
> patched: 10.35sec
>
> The improvement is less compared with the sequential write case, since
> we omit only half of the pages dirtied after snapshot command.
>
> Signed-off-by: Ethan Lien <ethanl...@synology.com>
> ---
> fs/btrfs/btrfs_inode.h | 1 +
> fs/btrfs/ctree.h | 2 +-
> fs/btrfs/extent_io.c | 16 ++++++++++++++--
> fs/btrfs/inode.c | 10 ++++++----
> fs/btrfs/ioctl.c | 2 +-
> 5 files changed, 23 insertions(+), 8 deletions(-)
>
> diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
> index 1343ac57b438..4182bfbb56be 100644
> --- a/fs/btrfs/btrfs_inode.h
> +++ b/fs/btrfs/btrfs_inode.h
> @@ -29,6 +29,7 @@ enum {
> BTRFS_INODE_IN_DELALLOC_LIST,
> BTRFS_INODE_READDIO_NEED_LOCK,
> BTRFS_INODE_HAS_PROPS,
> + BTRFS_INODE_TAGGED_FLUSH,
> };
>
> /* in memory btrfs inode */
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 2cddfe7806a4..82682da5a40d 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -3155,7 +3155,7 @@ int btrfs_truncate_inode_items(struct
> btrfs_trans_handle *trans,
> struct inode *inode, u64 new_size,
> u32 min_type);
>
> -int btrfs_start_delalloc_inodes(struct btrfs_root *root);
> +int btrfs_start_delalloc_snapshot(struct btrfs_root *root);
> int btrfs_start_delalloc_roots(struct btrfs_fs_info *fs_info, int nr);
> int btrfs_set_extent_delalloc(struct inode *inode, u64 start, u64 end,
> unsigned int extra_bits,
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 4dd6faab02bb..c21d8a0e010a 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -3928,12 +3928,24 @@ static int extent_write_cache_pages(struct
> address_space *mapping,
> range_whole = 1;
> scanned = 1;
> }
> - if (wbc->sync_mode == WB_SYNC_ALL)
> +
> + /*
> + * We don't care if we are the one who set BTRFS_INODE_TAGGED_FLUSH in
> + * start_delalloc_inodes(). We do the tagged writepage as long as we are
> + * the first one who do the filemap_flush() on this inode.
> + */
> + if (range_whole && wbc->nr_to_write == LONG_MAX &&
> + wbc->sync_mode == WB_SYNC_NONE &&
> + test_and_clear_bit(BTRFS_INODE_TAGGED_FLUSH,
> + &BTRFS_I(inode)->runtime_flags))
Actually this check can be simplified to:
range_whole && test_and_clear_bit. filemap_flush triggers range_whole =
1 and then you care about TAGGED_FLUSH (or w/e it's going to be named)
to be set. The nr_to_write && syncmode just make it a tad more difficult
to reason about the code.
> + wbc->tagged_writepages = 1;
> +
> + if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
> tag = PAGECACHE_TAG_TOWRITE;
> else
> tag = PAGECACHE_TAG_DIRTY;
> retry:
> - if (wbc->sync_mode == WB_SYNC_ALL)
> + if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
> tag_pages_for_writeback(mapping, index, end);
> done_index = index;
> while (!done && !nr_to_write_done && (index <= end) &&
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 3ea5339603cf..3df3cbbe91c5 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -9975,7 +9975,7 @@ static struct btrfs_delalloc_work
> *btrfs_alloc_delalloc_work(struct inode *inode
> * some fairly slow code that needs optimization. This walks the list
> * of all the inodes with pending delalloc and forces them to disk.
> */
> -static int start_delalloc_inodes(struct btrfs_root *root, int nr)
> +static int start_delalloc_inodes(struct btrfs_root *root, int nr, int
> snapshot)
> {
> struct btrfs_inode *binode;
> struct inode *inode;
> @@ -10003,6 +10003,8 @@ static int start_delalloc_inodes(struct btrfs_root
> *root, int nr)
> }
> spin_unlock(&root->delalloc_lock);
>
> + if (snapshot)
> + set_bit(BTRFS_INODE_TAGGED_FLUSH,
> &binode->runtime_flags);
> work = btrfs_alloc_delalloc_work(inode);
> if (!work) {
> iput(inode);
> @@ -10036,7 +10038,7 @@ static int start_delalloc_inodes(struct btrfs_root
> *root, int nr)
> return ret;
> }
>
> -int btrfs_start_delalloc_inodes(struct btrfs_root *root)
> +int btrfs_start_delalloc_snapshot(struct btrfs_root *root)
> {
> struct btrfs_fs_info *fs_info = root->fs_info;
> int ret;
> @@ -10044,7 +10046,7 @@ int btrfs_start_delalloc_inodes(struct btrfs_root
> *root)
> if (test_bit(BTRFS_FS_STATE_ERROR, &fs_info->fs_state))
> return -EROFS;
>
> - ret = start_delalloc_inodes(root, -1);
> + ret = start_delalloc_inodes(root, -1, 1);
> if (ret > 0)
> ret = 0;
> return ret;
> @@ -10073,7 +10075,7 @@ int btrfs_start_delalloc_roots(struct btrfs_fs_info
> *fs_info, int nr)
> &fs_info->delalloc_roots);
> spin_unlock(&fs_info->delalloc_root_lock);
>
> - ret = start_delalloc_inodes(root, nr);
> + ret = start_delalloc_inodes(root, nr, 0);
> btrfs_put_fs_root(root);
> if (ret < 0)
> goto out;
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index d60b6caf09e8..d1293b6c31f6 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -775,7 +775,7 @@ static int create_snapshot(struct btrfs_root *root,
> struct inode *dir,
> wait_event(root->subv_writers->wait,
> percpu_counter_sum(&root->subv_writers->counter) == 0);
>
> - ret = btrfs_start_delalloc_inodes(root);
> + ret = btrfs_start_delalloc_snapshot(root);
> if (ret)
> goto dec_and_free;
>
>
