linux-ext4 - Re: do not disable ext4 discards on first discard failure? [was: Re: dm snapshot: ignore discards issued to the snapshot-origin target]

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Date:	Mon, 2 May 2011 10:48:23 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Lukas Czerner <lczerner@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>,
	Christoph Hellwig <hch@...radead.org>, DarkNovaNick@...il.com,
	linux-lvm@...hat.com, linux-ext4@...r.kernel.org,
	Alasdair G Kergon <agk@...hat.com>,
	device-mapper development <dm-devel@...hat.com>
Subject: Re: do not disable ext4 discards on first discard failure? [was: Re:
 dm snapshot: ignore discards issued to the snapshot-origin target]

On Mon, May 02, 2011 at 09:47:16AM -0500, Eric Sandeen wrote:
> I still think we need to stick with the simple rule: "EOPNOTSUPP returned for a particular bio means that it is not supported for that particular bio" - I don't know what else we can do, without creating an ambiguity...
> 
> This does, however, suck for the layer calling in to a complex device.
> 
> What is the overhead for sending discard bios down to a device that does not support it?

For a DM-like device there is very little overhead.  The bio gets
dispatched to make_request and failed in there almost immediately.

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