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, 02 May 2011 09:47:16 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Lukas Czerner <lczerner@...hat.com>
CC:	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 5/2/11 8:05 AM, Lukas Czerner wrote:
> On Mon, 2 May 2011, Mike Snitzer wrote:

...

>> The blkdev_issue_discard() change you propose could be fine (mask
>> EOPNOTSUPP return if device advertises support for discards) -- though
>> Eric said we shouldn't ever say we did something when we didn't.
> 
> Exactly, so we should not say that it is not supported when it is, but
> we just hit the "wrong" part of the device:) I would just very much like
> to keep the abstraction of having one consistent device underneath the
> file system and not deal with several devices, or regions with different
> behaviour in the file system itself (let the pixies underneath deal with
> that, after all not all of us are btrfs:))

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?

-Eric
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