Re: [dm-devel] [Lsf-pc] [LSF/MM TOPIC] a few storage topics

[Chris Mason <chris mason oracle com>]

On Tue, Jan 24, 2012 at 01:40:54PM -0500, Christoph Hellwig wrote:
> On Tue, Jan 24, 2012 at 01:05:50PM -0500, Jeff Moyer wrote:
> > - buffered writes and buffered O_SYNC writes, all 1MB block size show 4k
> >   I/Os passed down to the I/O scheduler
> > - buffered 1MB reads are a little better, typically in the 128k-256k
> >   range when they hit the I/O scheduler.
> > 
> > ext4:
> > - buffered writes: 512K I/Os show up at the elevator
> > - buffered O_SYNC writes: data is again 512KB, journal writes are 4K
> > - buffered 1MB reads get down to the scheduler in 128KB chunks
> > 
> > xfs:
> > - buffered writes: 1MB I/Os show up at the elevator
> > - buffered O_SYNC writes: 1MB I/Os
> > - buffered 1MB reads: 128KB chunks show up at the I/O scheduler
> > 
> > So, ext4 is doing better than ext3, but still not perfect.  xfs is
> > kicking ass for writes, but reads are still split up.
> 
> All three filesystems use the generic mpages code for reads, so they
> all get the same (bad) I/O patterns.  Looks like we need to fix this up
> ASAP.

Can you easily run btrfs through the same rig?  We don't use mpages and
I'm curious.

-chris