linux-kernel - Re: [PATCH, RFC] xfs: batched discard support

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Message-ID: <4A8D60C1.6000809@redhat.com>
Date:	Thu, 20 Aug 2009 10:42:09 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Mark Lord <liml@....ca>
CC:	Ingo Molnar <mingo@...e.hu>, Christoph Hellwig <hch@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	xfs@....sgi.com, linux-fsdevel@...r.kernel.org,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	jens.axboe@...cle.com,
	IDE/ATA development list <linux-ide@...r.kernel.org>,
	Neil Brown <neilb@...e.de>
Subject: Re: [PATCH, RFC] xfs: batched discard support

On 08/20/2009 10:38 AM, Mark Lord wrote:
> Ric Wheeler wrote:
>>
>> Note that returning consistent data is critical for devices that are
>> used in a RAID group since you will need each RAID block that is used
>> to compute the parity to continue to return the same data until you
>> overwrite it with new data :-)
>>
>> If we have a device that does not support this (or is misconfigured
>> not to do this), we should not use those devices in an MD group & do
>> discard against it...
> ..
>
> Well, that's a bit drastic. But the RAID software should at least
> not issue TRIM commands in ignorance of such.

If the storage can return different data in a sequence of READ requests of the 
same sector (with no writes), there is nothing RAID could do. It would see total 
garbage...

> Would it still be okay to do the TRIMs when the entire parity stripe
> (across all members) is being discarded? (As opposed to just partial
> data there being dropped)

This should be safe if the MD bitmaps would prevent us from trying to 
READ/regenerate parity for that stripe...

ric

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