Btrfs: avoid syncing log in the fast fsync path when not necessary
Commit 3a8b36f37806 ("Btrfs: fix data loss in the fast fsync path") added
a performance regression for that causes an unnecessary sync of the log
trees (fs/subvol and root log trees) when 2 consecutive fsyncs are done
against a file, without no writes or any metadata updates to the inode in
between them and if a transaction is committed before the second fsync is
called.
Huang Ying reported this to lkml (https://lkml.org/lkml/2015/3/18/99)
after a test sysbench test that measured a -62% decrease of file io
requests per second for that tests' workload.
The test is:
echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
echo performance > /sys/devices/system/cpu/cpu1/cpufreq/scaling_governor
echo performance > /sys/devices/system/cpu/cpu2/cpufreq/scaling_governor
echo performance > /sys/devices/system/cpu/cpu3/cpufreq/scaling_governor
mkfs -t btrfs /dev/sda2
mount -t btrfs /dev/sda2 /fs/sda2
cd /fs/sda2
for ((i = 0; i < 1024; i++)); do fallocate -l 67108864 testfile.$i; done
sysbench --test=fileio --max-requests=0 --num-threads=4 --max-time=600 \
--file-test-mode=rndwr --file-total-size=68719476736 --file-io-mode=sync \
--file-num=1024 run
A test on kvm guest, running a debug kernel gave me the following results:
Without 3a8b36f378060d: 16.01 reqs/sec
With 3a8b36f378060d: 3.39 reqs/sec
With 3a8b36f378060d and this patch: 16.04 reqs/sec
Reported-by: Huang Ying <ying.huang@intel.com>
Tested-by: Huang, Ying <ying.huang@intel.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index b072e17..795d754 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1868,6 +1868,7 @@
struct btrfs_log_ctx ctx;
int ret = 0;
bool full_sync = 0;
+ const u64 len = end - start + 1;
trace_btrfs_sync_file(file, datasync);
@@ -1896,7 +1897,7 @@
* all extents are persisted and the respective file extent
* items are in the fs/subvol btree.
*/
- ret = btrfs_wait_ordered_range(inode, start, end - start + 1);
+ ret = btrfs_wait_ordered_range(inode, start, len);
} else {
/*
* Start any new ordered operations before starting to log the
@@ -1968,8 +1969,10 @@
*/
smp_mb();
if (btrfs_inode_in_log(inode, root->fs_info->generation) ||
- (full_sync && BTRFS_I(inode)->last_trans <=
- root->fs_info->last_trans_committed)) {
+ (BTRFS_I(inode)->last_trans <=
+ root->fs_info->last_trans_committed &&
+ (full_sync ||
+ !btrfs_have_ordered_extents_in_range(inode, start, len)))) {
/*
* We'v had everything committed since the last time we were
* modified so clear this flag in case it was set for whatever