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

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Message-ID: <4A8D5442.1000302@redhat.com>
Date:	Thu, 20 Aug 2009 09:48:50 -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 08/19/2009 09:39 PM, Mark Lord wrote:
> [resending, after fixing the Cc: list; somebody trimmed it earlier]
>
> Jamie Lokier wrote:
> ..
>> I don't remember - does TRIM guarantee the blocks read zeros afterwards?
> ..
>
> No, it doesn't.
>
> A drive can optionally support "deterministic TRIM", whereby it will 
> return
> consistent data for any given trimmed sector afterwards, but that 
> doesn't mean zeros.
>
> -ml

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...

ric

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