linux-kernel - Re: SATA exceptions with 2.6.20-rc5

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Message-ID: <45AC2F99.3040209@garzik.org>
Date:	Mon, 15 Jan 2007 20:51:21 -0500
From:	Jeff Garzik <jeff@...zik.org>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	Robert Hancock <hancockr@...w.ca>,
	Björn Steinbrink 
	<B.Steinbrink@....de>, linux-kernel@...r.kernel.org,
	htejun@...il.com
Subject: Re: SATA exceptions with 2.6.20-rc5

Jens Axboe wrote:
> On Mon, Jan 15 2007, Jeff Garzik wrote:
>> Jens Axboe wrote:
>>> I'd be surprised if the device would not obey the 7 second timeout rule
>>> that seems to be set in stone and not allow more dirty in-drive cache
>>> than it could flush out in approximately that time.
>> AFAIK Windows flush-cache timeout is 30 seconds, not 7 as with other 
>> commands...
> 
> Ok, 7 seconds for FLUSH_CACHE would have been nice for us too though, as
> it would pretty much guarentee lower latencies for random writes and
> write back caching. The concern is the barrier code, of course. I guess
> I should do some timings on potential worst case patterns some day. Alan
> may have done that sometime in the past, iirc.

FWIW:  According to the drive guys (Eric M, among others), FLUSH CACHE 
will "probably" be under 30 seconds, but pathological cases might even 
extend beyond that.

Definitely more than 7 seconds in less-than-pathological cases, 
unfortunately...

The SCSI layer /should/ already take this (30 second timeout) into 
account, for SYNCHRONIZE CACHE (and thus FLUSH CACHE for libata) but I'm 
too slack to check at the moment.

	Jeff



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