Re: Linux 2.6.29

Mark Lord
2009-03-30T17:30:22Z

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).
>
> You'll have a much easier time getting long latencies when seeks take tens
> of ms, and the platter rotates at some pitiful 3600rpm (ok, I guess those
> drives are hard to find these days - I guess 4200rpm is the norm even for
> 1.8" laptop harddrives).
>
> And also - this is probably obvious to you, but it might not be
> immediately obvious to everybody - make sure that you do have TCQ going,
> and at full depth. If the drive supports TCQ (and they all do, these days)
> it is quite possible that the drive firmware basically limits the write
> caching to one segment per TCQ entry (or at least to something smallish).
..

Oh yes, absolute -- I tried with and without NCQ (the SATA replacement
for old-style TCQ), and with varying NCQ queue depths. No luck keeping
the darned thing busy flushing afterwards for anything more than
perhaps a few hundred millseconds. I wasn't really interested in anything
under a second, so I didn't measure it exactly though.

The older and/or slower notebook drives (4200rpm) tend to have smaller
onboard caches, too. Which makes them difficult to fill.

I suspect I'd have much better "luck" with a slow-ish SSD that has
a largish write cache. Dunno if those exist, and they'll have to get
cheaper before I pick one up to deliberately bash on. :)

Cheers