Re: [PATCH RFC] vfs: add a O_NOMTIME flag

[Trond Myklebust <trond.myklebust@primarydata.com>]

Hi Zach,

On Wed, May 6, 2015 at 6:00 PM, Zach Brown <zab@redhat.com> wrote:
>
> Add the O_NOMTIME flag which prevents mtime from being updated which can
> greatly reduce the IO overhead of writes to allocated and initialized
> regions of files.
>
> ceph servers can have loads where they perform O_DIRECT overwrites of
> allocated file data and then sync to make sure that the O_DIRECT writes
> are flushed from write caches.  If the writes dirty the inode with mtime
> updates then the syncs also write out the metadata needed to track the
> inodes which can add significant iop and latency overhead.
>
> The ceph servers don't use mtime at all.  They're using the local file
> system as a backing store and any backups would be driven by their upper
> level ceph metadata.  For ceph, slow IO from mtime updates in the file
> system is as daft as if we had block devices slowing down IO for
> per-block write timestamps that file systems never use.
>
> In simple tests a O_DIRECT|O_NOMTIME overwriting write followed by a
> sync went from 2 serial write round trips to 1 in XFS and from 4 serial
> IO round trips to 1 in ext4.
>
> file_update_time() checks for O_NOMTIME and aborts the update if it's
> set, just like the current check for the in-kernel inode flag
> S_NOCMTIME.  I didn't update any other mtime update sites. They could be
> added as we decide that it's appropriate to do so.
>
> I opted not to name the flag O_NOCMTIME because I didn't want the name
> to imply that ctime updates would be prevented for other inode changes
> like updating i_size in truncate.  Not updating ctime is a side-effect
> of removing mtime updates when it's the only thing changing in the
> inode.
>
> The criteria for using O_NOMTIME is the same as for using O_NOATIME:
> owning the file or having the CAP_FOWNER capability.  If we're not
> comfortable allowing owners to prevent mtime/ctime updates then we
> should add a tunable to allow O_NOMTIME.  Maybe a mount option?
>

Just out of curiosity, if you need to modify the application anyway,
why wouldn't use of fdatasync() when flushing be able to offer a
similar performance boost?

Cheers
  Trond