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 generic write_cache_pages(): we quickly
tag all dirty pages with a TOWRITE tag, then do the hard work of
writepage only on those pages with TOWRITE tag, so we 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 <ethanlien@xxxxxxxxxxxx>
---
V2:
Add more details in commit message.
rename BTRFS_INODE_TAGGED_FLUSH to BTRFS_INODE_SNAPSHOT_FLUSH.
remove unnecessary sync_mode check.
start_delalloc_inodes use boolean argument.
fs/btrfs/btrfs_inode.h | 1 +
fs/btrfs/ctree.h | 2 +-
fs/btrfs/extent_io.c | 14 ++++++++++++--
fs/btrfs/inode.c | 10 ++++++----
fs/btrfs/ioctl.c | 2 +-
5 files changed, 21 insertions(+), 8 deletions(-)
diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
index 1343ac57b438..ffc9a1c77375 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_SNAPSHOT_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..93f2e413535d 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -3928,12 +3928,22 @@ static int extent_write_cache_pages(struct address_space *mapping,
range_whole = 1;
scanned = 1;
}
- if (wbc->sync_mode == WB_SYNC_ALL)
+
+ /*
+ * We do the tagged writepage as long as the snapshot flush bit is set
+ * and we are the first one who do the filemap_flush() on this inode.
+ */
+ if (range_whole && wbc->nr_to_write == LONG_MAX &&
+ test_and_clear_bit(BTRFS_INODE_SNAPSHOT_FLUSH,
+ &BTRFS_I(inode)->runtime_flags))
+ 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..593445d122ed 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, bool 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_SNAPSHOT_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, true);
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, false);
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;
--
2.19.1
