In nocow path, we check if the extent is snapshotted in
btrfs_cross_ref_exist(). We can do the similar check earlier and avoid
unnecessary search into extent tree.
A fio test on a Intel D-1531, 16GB RAM, SSD RAID-5 machine as follows:
[global]
group_reporting
time_based
thread=1
ioengine=libaio
bs=4k
iodepth=32
size=64G
runtime=180
numjobs=8
rw=randwrite
[file1]
filename=/mnt/nocow/testfile
IOPS result: unpatched patched
1 fio round: 46670 46958
snapshot
2 fio round: 51826 54498
3 fio round: 59767 61289
After snapshot, the first fio get about 5% performance gain. As we
continually write to the same file, all writes will resume to nocow mode
and eventually we have no performance gain.
Signed-off-by: Ethan Lien <ethanlien@xxxxxxxxxxxx>
---
V2:
Add comment and performance test.
fs/btrfs/inode.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index d241285a0d2a..177630337108 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1373,6 +1373,13 @@ static noinline int run_delalloc_nocow(struct inode *inode,
btrfs_file_extent_encryption(leaf, fi) ||
btrfs_file_extent_other_encoding(leaf, fi))
goto out_check;
+ /*
+ * We can skip the checking of generation of
+ * extent item in btrfs_cross_ref_exist().
+ */
+ if (btrfs_file_extent_generation(leaf, fi) <=
+ btrfs_root_last_snapshot(&root->root_item))
+ goto out_check;
if (extent_type == BTRFS_FILE_EXTENT_REG && !force)
goto out_check;
if (btrfs_extent_readonly(fs_info, disk_bytenr))
@@ -7368,6 +7375,14 @@ noinline int can_nocow_extent(struct inode *inode, u64 offset, u64 *len,
btrfs_file_extent_other_encoding(leaf, fi))
goto out;
+ /*
+ * We can skip the checking of generation of
+ * extent item in btrfs_cross_ref_exist().
+ */
+ if (btrfs_file_extent_generation(leaf, fi) <=
+ btrfs_root_last_snapshot(&root->root_item))
+ goto out;
+
backref_offset = btrfs_file_extent_offset(leaf, fi);
if (orig_start) {
--
2.17.0
--
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