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btrfs: lift some btrfs_cross_ref_exist checks in nocow path 

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@synology.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update comments ]
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Ethan Lien 2018-05-17 14:58:29 +08:00 committed by David Sterba
parent d19577912d
commit 78d4295b1e
1 changed files with 15 additions and 0 deletions
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15
fs/btrfs/inode.c
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@ -1370,6 +1370,13 @@ next_slot:
btrfs_file_extent_encryption(leaf, fi) ||
btrfs_file_extent_other_encoding(leaf, fi))
goto out_check;
/*
* Do the same check as in btrfs_cross_ref_exist but
* without the unnecessary search.
*/
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))
@ -7322,6 +7329,14 @@ noinline int can_nocow_extent(struct inode *inode, u64 offset, u64 *len,
btrfs_file_extent_other_encoding(leaf, fi))
goto out;
/*
* Do the same check as in btrfs_cross_ref_exist but without the
* unnecessary search.
*/
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) {