Since commit d4682ba0 ("Btrfs: sync log after logging new name") we
started to commit logs, and fallback to transaction commits when we failed
to log the new names or commit the logs, after link and rename operations
when the target inodes (or their parents) were previously logged in the
current transaction. This was to avoid losing directories despite an
explicit fsync on them when they are ancestors of some inode that got a
new named logged, due to a link or rename operation. However that adds the
cost of starting IO and waiting for it to complete, which can cause higher
latencies for applications.

Instead of doing that, just make sure that when we log a new name for an
inode we don't mark any of its ancestors as logged, so that if any one
does an fsync against any of them, without doing any other change on them,
the fsync commits the log. This way we only pay the cost of a log commit
(or a transaction commit if something goes wrong or a new block group was
created) if the application explicitly asks to fsync any of the parent
directories.

Using dbench, which mixes several filesystems operations including renames,
revealed some significant latency gains. The following script that uses
dbench was used to test this:

  #!/bin/bash

  DEV=/dev/nvme0n1
  MNT=/mnt/btrfs
  MOUNT_OPTIONS="-o ssd -o space_cache=v2"
  MKFS_OPTIONS="-m single -d single"
  THREADS=16

  echo "performance" | tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
  mkfs.btrfs -f $MKFS_OPTIONS $DEV
  mount $MOUNT_OPTIONS $DEV $MNT

  dbench -t 300 -D $MNT $THREADS

  umount $MNT

The test was run on bare metal, no virtualization, on a box with 12 cores
(Intel i7-8700), 64Gb of RAM and using a NVMe device, with a kernel
configuration that is the default of typical distributions (debian in this
case), without debug options enabled (kasan, kmemleak, slub debug, debug
of page allocations, lock debugging, etc).

Results before this patch:

 Operation      Count    AvgLat    MaxLat
 ----------------------------------------
 NTCreateX    10750455     0.011   155.088
 Close         7896674     0.001     0.243
 Rename         455222     2.158  1101.947
 Unlink        2171189     0.067   121.638
 Deltree           256     2.425     7.816
 Mkdir             128     0.002     0.003
 Qpathinfo     9744323     0.006    21.370
 Qfileinfo     1707092     0.001     0.146
 Qfsinfo       1786756     0.001    11.228
 Sfileinfo      875612     0.003    21.263
 Find          3767281     0.025     9.617
 WriteX        5356924     0.011   211.390
 ReadX        16852694     0.003     9.442
 LockX           35008     0.002     0.119
 UnlockX         35008     0.001     0.138
 Flush          753458     4.252  1102.249

Throughput 1128.35 MB/sec  16 clients  16 procs  max_latency=1102.255 ms

Results after this patch:

16 clients, after

 Operation      Count    AvgLat    MaxLat
 ----------------------------------------
 NTCreateX    11471098     0.012   448.281
 Close         8426396     0.001     0.925
 Rename         485746     0.123   267.183
 Unlink        2316477     0.080    63.433
 Deltree           288     2.830    11.144
 Mkdir             144     0.003     0.010
 Qpathinfo    10397420     0.006    10.288
 Qfileinfo     1822039     0.001     0.169
 Qfsinfo       1906497     0.002    14.039
 Sfileinfo      934433     0.004     2.438
 Find          4019879     0.026    10.200
 WriteX        5718932     0.011   200.985
 ReadX        17981671     0.003    10.036
 LockX           37352     0.002     0.076
 UnlockX         37352     0.001     0.109
 Flush          804018     5.015   778.033

Throughput 1201.98 MB/sec  16 clients  16 procs  max_latency=778.036 ms
(+6.5% throughput, -29.4% max latency, -75.8% rename latency)

Test case generic/498 from fstests tests the scenario that the previously
mentioned commit fixed.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>