linux-kernel - Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

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Message-ID: <18010.12880.258773.95448@notabene.brown>
Date:	Mon, 28 May 2007 11:37:20 +1000
From:	Neil Brown <neilb@...e.de>
To:	"Stefan Bader" <Stefan.Bader@...ibm.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, linux-raid@...r.kernel.org,
	"Jens Axboe" <jens.axboe@...cle.com>, "David Chinner" <dgc@....com>
Subject: Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

On Friday May 25, Stefan.Bader@...ibm.com wrote:
> 2007/5/25, Neil Brown <neilb@...e.de>:
> >  - Are there other bit that we could handle better?
> >     BIO_RW_FAILFAST?  BIO_RW_SYNC?  What exactly do they mean?
> >
> BIO_RW_FAILFAST: means low-level driver shouldn't do much (or no)
> error recovery. Mainly used by mutlipath targets to avoid long SCSI
> recovery. This should just be propagated when passing requests on.

Is it "much" or "no"?
Would it be reasonable to use this for reads from a non-degraded
raid1?  What about writes?

What I would really like is some clarification on what sort of errors
get retried, how often, and how much timeout there is..

And does the 'error' code returned in ->bi_end_io allow us to
differentiate media errors from other errors yet?

> 
> BIO_RW_SYNC: means this is a bio of a synchronous request. I don't
> know whether there are more uses to it but this at least causes queues
> to be flushed immediately instead of waiting for more requests for a
> short time. Should also just be passed on. Otherwise performance gets
> poor since something above will rather wait for the current
> request/bio to complete instead of sending more.

Yes, this one is pretty straight forward.. I mentioned it more as a
reminder to my self that I really should support it in raid5 :-(

NeilBrown
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