Date:	Mon,  8 Dec 2014 23:04:27 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Linux Filesystem Development List <linux-fsdevel@...r.kernel.org>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Theodore Ts'o <tytso@....edu>
Subject: [PATCH-v7 0/3] add support for a lazytime mount option

This is an updated version of what had originally been an
ext4-specific patch which significantly improves performance by lazily
writing timestamp updates (and in particular, mtime updates) to disk.
The in-memory timestamps are always correct, but they are only written
to disk when required for correctness.

This provides a huge performance boost for ext4 due to how it handles
journalling, but it's valuable for all file systems running on flash
storage or drive-managed SMR disks by reducing the metadata write
load.  So upon request, I've moved the functionality to the VFS layer.
Once the /sbin/mount program adds support for MS_LAZYTIME, all file
systems should be able to benefit from this optimization.

There is still an ext4-specific optimization, which may be applicable
for other file systems which store more than one inode in a block, but
it will require file system specific code.  It is purely optional,
however.

For people interested seeing how timestamp updates are held back, the
following example commands to enable the tracepoints debugging may be
helpful:

  mount -o remount,lazytime /
  cd /sys/kernel/debug/tracing
  echo 1 > events/writeback/writeback_lazytime/enable
  echo 1 > events/writeback/writeback_lazytime_iput/enable
  echo "state & 2048" > events/writeback/writeback_dirty_inode_enqueue/filter
  echo 1 > events/writeback/writeback_dirty_inode_enqueue/enable
  echo 1 > events/ext4/ext4_other_inode_update_time/enable
  cat trace_pipe

You can also see how many lazytime inodes are in memory by looking in
/sys/kernel/debug/bdi/<bdi>/stats

Changes since -v6:
   - Add a new tracepoint writeback_dirty_inode_enqueue
   - Move generic handling of update_time() to generic_update_time(),
     so filesystems can more easily hook or modify update_time()
   - The file system's dirty_inode() will now always get called with
     I_DIRTY_TIME when the inode time is updated.   (I_DIRTY_SYNC will
     also be set if the inode should be updated right away.)   This allows
     file systems such as XFS to update its on-disk copy of the inode if
     I_DIRTY_TIME is set.

Changes since -v5:
   - Tweak move_expired_inodes to handle sync() and syncfs(), and drop
     flush_sb_dirty_time().
   - Move logic for handling the b_dirty_time list into
     __mark_inode_dirty().
   - Move I_DIRTY back to its original definition, and use I_DIRTY_ALL
     for I_DIRTY plus I_DIRTY_TIME.
   - Fold some patches together to make the first patch easier to
     review (and modify/update).
   - Use the pre-existing writeback tracepoints instead of creating a new
     fs tracepoints.

Changes since -v4:
   - Fix ext4 optimization so it does not need to increment (and more
     problematically, decrement) the inode reference count
   - Per Christoph's suggestion, drop support for btrfs and xfs for now,
     issues with how btrfs and xfs handle dirty inode tracking.  We can add
     btrfs and xfs support back later or at the end of this series if we
     want to revisit this decision.
   - Miscellaneous cleanups

Changes since -v3:
   - inodes with I_DIRTY_TIME set are placed on a new bdi list,
        b_dirty_time.  This allows filesystem-level syncs to more
        easily iterate over those inodes that need to have their
        timestamps written to disk.
   - dirty timestamps will be written out asynchronously on the final
        iput, instead of when the inode gets evicted.
   - separate the definition of the new function
        find_active_inode_nowait() to a separate patch
   - create separate flag masks: I_DIRTY_WB and I_DIRTY_INODE, which
       indicate whether the inode needs to be on the write back lists,
       or whether the inode itself is dirty, while I_DIRTY means any one
       of the inode dirty flags are set.  This simplifies the fs
       writeback logic which needs to test for different combinations of
       the inode dirty flags in different places.

Changes since -v2:
   - If update_time() updates i_version, it will not use lazytime (i..e,
       the inode will be marked dirty so the change will be persisted on to
       disk sooner rather than later).  Yes, this eliminates the
       benefits of lazytime if the user is experting the file system via
       NFSv4.  Sad, but NFS's requirements seem to mandate this.
   - Fix time wrapping bug 49 days after the system boots (on a system
        with a 32-bit jiffies).   Use get_monotonic_boottime() instead.
   - Clean up type warning in include/tracing/ext4.h
   - Added explicit parenthesis for stylistic reasons    
   - Added an is_readonly() inode operations method so btrfs doesn't
       have to duplicate code in update_time().

Changes since -v1:
   - Added explanatory comments in update_time() regarding i_ts_dirty_days
   - Fix type used for days_since_boot
   - Improve SMP scalability in update_time and ext4_update_other_inodes_time
   - Added tracepoints to help test and characterize how often and under
         what circumstances inodes have their timestamps lazily updated

Theodore Ts'o (3):
  vfs: add support for a lazytime mount option
  vfs: add find_inode_nowait() function
  ext4: add optimization for the lazytime mount option

 fs/ext4/inode.c                  |  71 ++++++++++++++++++++++++--
 fs/ext4/super.c                  |  10 ++++
 fs/fs-writeback.c                |  64 ++++++++++++++++++-----
 fs/gfs2/file.c                   |   4 +-
 fs/inode.c                       | 106 +++++++++++++++++++++++++++++++++------
 fs/jfs/file.c                    |   2 +-
 fs/libfs.c                       |   2 +-
 fs/proc_namespace.c              |   1 +
 fs/sync.c                        |   8 +++
 include/linux/backing-dev.h      |   1 +
 include/linux/fs.h               |  10 ++++
 include/trace/events/ext4.h      |  30 +++++++++++
 include/trace/events/writeback.h |  60 +++++++++++++++++++++-
 include/uapi/linux/fs.h          |   4 +-
 mm/backing-dev.c                 |  10 +++-
 15 files changed, 344 insertions(+), 39 deletions(-)

-- 
2.1.0

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