linux-kernel - Re: [PATCH] loop: add discard support for loop devices

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Date:	Thu, 18 Aug 2011 20:59:25 +0200
From:	Milan Broz <mbroz@...hat.com>
To:	Lukas Czerner <lczerner@...hat.com>
CC:	Jeff Moyer <jmoyer@...hat.com>, linux-kernel@...r.kernel.org,
	achender@...ux.vnet.ibm.com, Jens Axboe <jaxboe@...ionio.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] loop: add discard support for loop devices

On 08/18/2011 05:49 PM, Lukas Czerner wrote:
> On Thu, 18 Aug 2011, Jeff Moyer wrote:
>> Seems you missed the bizarre case of configuring a loop device over top
>> of a block device.
> 
> Wow, that is a bizarre case I did not think about at all. But since it
> is so bizarre, do we even care ? The thing is that the only case where
> it would make a difference is if the loop device is put on top of block
> device which actually supports discard.
> 
> In order to fix that I would need to dig out the actual limits for that
> device and set that appropriately for the loop device. Is that worth it
> ? It is not like someone will ever do that (or should) :).

It is bizarre (and being device-mapper developer I surely know better way :-)
but people are still using that.

Historically one of the use of underlying block device was cryptoloop, but here
I think it should be completely deprecated (cryptsetup can handle all old loop
modes as well and default modes for cryptoloop are not safe).
[Can we finally remove crypto loop option it from kernel? ... ok, just tried:)]

There is also out of tree loop-aes based on heavily patched loop device
which usually uses block device underneath
(cryptsetup already can handle all loop-aes modes as well).

Sometimes it is used with --offset parameter for some reason
(like linear device-mapper mapping).

So I do not care if you do not support discard here but please do not break
support for block device mapped through loop.

Thanks,
Milan
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