svn commit: r298002 - in head/sys: cam cam/ata cam/scsi conf dev/ahci

[Shawn Webb]

On Thu, Apr 14, 2016 at 04:04:27PM -0600, Warner Losh wrote:
> On Thu, Apr 14, 2016 at 3:54 PM, Dmitry Morozovsky <marck at rinet.ru> wrote:
> 
> > Warner,
> >
> > On Thu, 14 Apr 2016, Warner Losh wrote:
> >
> > > Author: imp
> > > Date: Thu Apr 14 21:47:58 2016
> > > New Revision: 298002
> > > URL: https://svnweb.freebsd.org/changeset/base/298002
> > >
> > > Log:
> > >   New CAM I/O scheduler for FreeBSD. The default I/O scheduler is the
> > same
> >
> > [snip]
> >
> > First, thanks so much for this quite a non-trivial work!
> > What are the ways to enable this instead of deafult, and what ar the
> > benefits
> > and drawbacks?
> 
> 
> You add CAM_NETFLIX_IOSCHED to your kernel config to enable it. Hmmm,
> looking at the diff, perhaps I should add that to LINT.
> 
> In production, we use it for three things. First, our scheduler keeps a lot
> more
> statistics than the default one. These statistics are useful for us knowing
> when
> a system is saturated and needs to shed load. Second, we favor reads over
> writes because our workload, as you might imagine, is a read mostly work
> load.
> Finally, in some systems, we throttle the write throughput to the SSDs. The
> SSDs
> we buy can do 300MB/s write while serving 400MB/s read, but only for short
> periods of time (long enough to do 10-20GB of traffic). After that, write
> performance
> drops, and read performance goes out the window. Experiments have shown that
> if we limit the write speed to no more than 30MB/s or so, then the garbage
> collection the drive is doing won't adversely affect the read latency /
> performance.

Going on a tangent here, but related:

As someone who is just barely stepping into the world of benchmarks and
performance metrics, can you shed some light as to how you gained those
metrics? I'd be extremely interested to learn.

Thanks,

-- 
Shawn Webb
HardenedBSD

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