Re: Linux 2.6.29

Pasi Kärkkäinen
2009-03-30T19:14:17Z

On Mon, Mar 30, 2009 at 01:57:12PM -0400, Chris Mason wrote:
> On Mon, 2009-03-30 at 09:58 -0700, Linus Torvalds wrote:
> >
> > On Mon, 30 Mar 2009, Mark Lord wrote:
> > >
> > > I spent an entire day recently, trying to see if I could significantly fill
> > > up the 32MB cache on a 750GB Hitach SATA drive here.
> > >
> > > With deliberate/random write patterns, big and small, near and far,
> > > I could not fill the drive with anything approaching a full second
> > > of latent write-cache flush time.
> > >
> > > Not even close. Which is a pity, because I really wanted to do some testing
> > > related to a deep write cache. But it just wouldn't happen.
> > >
> > > I tried this again on a 16MB cache of a Seagate drive, no difference.
> > >
> > > Bummer. :)
> >
> > Try it with laptop drives. You might get to a second, or at least hundreds
> > of ms (not counting the spinup delay if it went to sleep, obviously). You
> > probably tested desktop drives (that 750GB Hitachi one is not a low end
> > one, and I assume the Seagate one isn't either).
>
> I had some fun trying things with this, and I've been able to reliably
> trigger stalls in write cache of ~60 seconds on my seagate 500GB sata
> drive. The worst I saw was 214 seconds.
>
> It took a little experimentation, and I had to switch to the noop
> scheduler (no idea why).
>

I remember cfq having a bug (or a feature?) that prevents queue depths
deeper than 1.. so with noop you get more ios to the queue.

-- Pasi