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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 10:42:09 -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: <4A8D5FDB.7080505@xxxxxx>
References: <20090816004705.GA7347@xxxxxxxxxxxxx> <20090819203916.GA25296@xxxxxxx> <4A8CA956.2060406@xxxxxx> <4A8D5442.1000302@xxxxxxxxxx> <4A8D5FDB.7080505@xxxxxx>
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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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