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Re: [PATCH, RFC] xfs: batched discard support

To: Mark Lord <liml@xxxxxx>
Subject: Re: [PATCH, RFC] xfs: batched discard support
From: Ric Wheeler <rwheeler@xxxxxxxxxx>
Date: Thu, 20 Aug 2009 09:48:50 -0400
Cc: Ingo Molnar <mingo@xxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>, Paul Mackerras <paulus@xxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-scsi@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, jens.axboe@xxxxxxxxxx, IDE/ATA development list <linux-ide@xxxxxxxxxxxxxxx>, Neil Brown <neilb@xxxxxxx>
In-reply-to: <4A8CA956.2060406@xxxxxx>
References: <20090816004705.GA7347@xxxxxxxxxxxxx> <20090819203916.GA25296@xxxxxxx> <4A8CA956.2060406@xxxxxx>
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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.


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...


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