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

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Message-ID: <alpine.LFD.2.00.1108182104390.22042@dhcp-27-109.brq.redhat.com>
Date:	Thu, 18 Aug 2011 21:08:46 +0200 (CEST)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Milan Broz <mbroz@...hat.com>
cc:	Lukas Czerner <lczerner@...hat.com>,
	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 Thu, 18 Aug 2011, Milan Broz wrote:

> 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.

I do not think that this is the case with my patch. Also, as you know using
discard on encrypted device is not a good idea.

Thanks!
-Lukas

> 
> Thanks,
> Milan
> 

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