• Jan Kara's avatar
    writeback: do not sync data dirtied after sync start · c4a391b5
    Jan Kara authored
    When there are processes heavily creating small files while sync(2) is
    running, it can easily happen that quite some new files are created
    between WB_SYNC_NONE and WB_SYNC_ALL pass of sync(2).  That can happen
    especially if there are several busy filesystems (remember that sync
    traverses filesystems sequentially and waits in WB_SYNC_ALL phase on one
    fs before starting it on another fs).  Because WB_SYNC_ALL pass is slow
    (e.g.  causes a transaction commit and cache flush for each inode in
    ext3), resulting sync(2) times are rather large.
    
    The following script reproduces the problem:
    
      function run_writers
      {
        for (( i = 0; i < 10; i++ )); do
          mkdir $1/dir$i
          for (( j = 0; j < 40000; j++ )); do
            dd if=/dev/zero of=$1/dir$i/$j bs=4k count=4 &>/dev/null
          done &
        done
      }
    
      for dir in "$@"; do
        run_writers $dir
      done
    
      sleep 40
      time sync
    
    Fix the problem by disregarding inodes dirtied after sync(2) was called
    in the WB_SYNC_ALL pass.  To allow for this, sync_inodes_sb() now takes
    a time stamp when sync has started which is used for setting up work for
    flusher threads.
    
    To give some numbers, when above script is run on two ext4 filesystems
    on simple SATA drive, the average sync time from 10 runs is 267.549
    seconds with standard deviation 104.799426.  With the patched kernel,
    the average sync time from 10 runs is 2.995 seconds with standard
    deviation 0.096.
    Signed-off-by: default avatarJan Kara <jack@suse.cz>
    Reviewed-by: default avatarFengguang Wu <fengguang.wu@intel.com>
    Reviewed-by: default avatarDave Chinner <dchinner@redhat.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    c4a391b5
fs-writeback.c 39.6 KB