From: Ric Wheeler <ric@emc.com>
To: Jeff Garzik <jeff@garzik.org>
Cc: Lars Ellenberg <Lars.Ellenberg@linbit.com>,
Ingo Molnar <mingo@redhat.com>,
Neil Brown <neilb@cse.unsw.edu.au>,
linux-raid@vger.kernel.org, Andrew Morton <akpm@osdl.org>,
Jens Axboe <axboe@suse.de>
Subject: Re: [patch] latency problem in md driver
Date: Fri, 22 Dec 2006 10:37:14 -0500 [thread overview]
Message-ID: <458BFBAA.3060805@emc.com> (raw)
In-Reply-To: <458BF799.6010703@garzik.org>
Jeff Garzik wrote:
> Lars Ellenberg wrote:
>> md raidX make_request functions strip off the BIO_RW_SYNC flag,
>> this introducing additional latency.
>>
>> below is a suggested patch for the raid1.c .
>> other suggested solutions would be to let the bio_clone do its work,
>> and not reassign thereby stripping off all flags.
>> at most strip off known unwanted flags (the BARRIER flag).
>
> It sounds like a major bug to strip the barrier flag. I quite
> understand that a barrier to a RAID device as a whole behaves
> differently from a barrier to an ATA or SCSI device, but that's no
> excuse to avoid the problem.
>
> If MD does not pass barriers, it is unilaterally dropping the "data made
> it to the media" guarantee.
>
> Jeff
Exactly right - if we do not pass the barrier request down to the
members of the RAID group, then we lose the data integrity needed.
Of course, in a RAID group, this will be introduce latency, but that is
the correct behavior.
ric
next prev parent reply other threads:[~2006-12-22 15:37 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-12-22 14:32 [patch] latency problem in md driver Lars Ellenberg
2006-12-22 15:19 ` Jeff Garzik
2006-12-22 15:37 ` Ric Wheeler [this message]
2006-12-22 15:40 ` Lars Ellenberg
2006-12-22 17:33 ` Lars Ellenberg
2006-12-22 20:40 ` Neil Brown
2006-12-26 13:09 ` Lars Ellenberg
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