• Filipe Manana's avatar
    Btrfs: avoid syncing log in the fast fsync path when not necessary · b659ef02
    Filipe Manana authored
    Commit 3a8b36f3 ("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 3a8b36f3:             16.01 reqs/sec
    With 3a8b36f3:                 3.39 reqs/sec
    With 3a8b36f3 and this patch: 16.04 reqs/sec
    Reported-by: default avatarHuang Ying <ying.huang@intel.com>
    Tested-by: default avatarHuang, Ying <ying.huang@intel.com>
    Signed-off-by: default avatarFilipe Manana <fdmanana@suse.com>
    Signed-off-by: default avatarChris Mason <clm@fb.com>
    b659ef02
file.c 76.2 KB