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

[Sage Weil]

On Wed, 6 May 2015, Zach Brown wrote:
> On Wed, May 06, 2015 at 03:19:13PM -0700, Sage Weil wrote:
> > On Wed, 6 May 2015, Trond Myklebust wrote:
> > > Hi Zach,
> > > 
> > > On Wed, May 6, 2015 at 6:00 PM, Zach Brown <zab@xxxxxxxxxx> 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?
> > 
> > Although fdatasync(2) doesn't have to update synchronously, it does 
> > eventually get written, and that can trigger lots of unwanted IO.
> 
> And the unwanted IO is per file.  Are there circumstances where the
> write:file ratio is small enough that dirty inode writes could start to
> add up to meaningful write amplification?

Yeah, exactly: in some not-so-uncommon workloads it's approaching 1:1.

sage
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