Re: [PATCH 0/4] (RESEND) ext3[34] barrier changes

[Jamie Lokier <jamie@shareable.org>]

Chris Mason wrote:
> On Sunday 18 May 2008, Andi Kleen wrote:
> > Andrew Morton <akpm@linux-foundation.org> writes:
> > > On Fri, 16 May 2008 14:02:46 -0500
> > >
> > > Eric Sandeen <sandeen@redhat.com> wrote:
> > >> A collection of patches to make ext3 & 4 use barriers by
> > >> default, and to call blkdev_issue_flush on fsync if they
> > >> are enabled.
> > >
> > > Last time this came up lots of workloads slowed down by 30% so I
> > > dropped the patches in horror.
> >
> > Didn't ext4 have some new checksum trick to avoid them?
> 
> I didn't think checksumming avoided barriers completely.  Just the barrier 
> before the commit block, not the barrier after.

A little optimisation note.

You don't need the barrier after in some cases, or it can be deferred
until a better time.  E.g. when the disk write cache is probably empty
(some time after write-idle), barrier flushes may take the same time
as NOPs.

This sequence:

    #1 write metadata to journal
    #1 write commit block (checksummed)
  BARRIER
    #1 write metadata in place
  ... time passes ...
    #2 write metadata to journal
    #2 write commit block (checksummed)
  BARRIER
    #2 write metadata in place
  ... time passes ...
    #3 write metadata to journal
    #3 write commit block (checksummed)
  BARRIER
    #3 write metadata in place

Can be rewritten as:

    #1 write metadata to journal
    #1 write commit block (checksummed)
  ... time passes ...
    #2 write metadata to journal
    #2 write commit block (checksummed)
  ... time passes ...
    #3 write metadata to journal
    #3 write commit block (checksummed)
  ... time passes ...
  BARRIER (probably instant).
    #1 write metadata in place
    #2 write metadata in place
    #3 write metadata in place

Provided some conditions hold.  All the metadata and all the journal
writes being non-overlapping I/O ranges would be sufficient.

What's more, barriers can be deferred past data=ordered in-place data
writes, although that's not always an optimisation.

-- Jamie