Re: filesystem benchmarking fun

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 16 May 2007 16:14:14 -0400
Chris Mason <chris.mason@oracle.com> wrote:

> On Wed, May 16, 2007 at 01:04:13PM -0700, Andrew Morton wrote:
> > > The good news is that if you let it run long enough, the times
> > > stabilize.  The bad news is:
> > > 
> > > create dir kernel-86 222MB in 15.85 seconds (14.03 MB/s)
> > > create dir kernel-87 222MB in 28.67 seconds (7.76 MB/s)
> > > create dir kernel-88 222MB in 18.12 seconds (12.27 MB/s)
> > > create dir kernel-89 222MB in 19.77 seconds (11.25 MB/s)
> > 
> > well hang on.  Doesn't this just mean that the first few runs were writing
> > into pagecache and the later ones were blocking due to dirty-memory limits?
> > 
> > Or do you have a sync in there?
> > 
> There's no sync,  but if you watch vmstat you can clearly see the log
> flushes, even when the overall create times are 11MB/s.  vmstat goes
> 30MB/s -> 4MB/s or less, then back up to 30MB/s.

How do you know that it is a log flush rather than, say, pdflush
hitting the blockdev inode and doing a big seeky write?