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

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Date:	Thu, 18 Aug 2011 21:20:58 +0200 (CEST)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Jens Axboe <jaxboe@...ionio.com>
cc:	Lukas Czerner <lczerner@...hat.com>, Milan Broz <mbroz@...hat.com>,
	Jeff Moyer <jmoyer@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"achender@...ux.vnet.ibm.com" <achender@...ux.vnet.ibm.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] loop: add discard support for loop devices

On Thu, 18 Aug 2011, Jens Axboe wrote:

> On 2011-08-18 21:08, Lukas Czerner wrote:
> > 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.
> 
> It's not a bizarre use case at all, so would be nice to support like we
> support anything else over a bdev as well. Your patch should not break
> it, so looks fine.
> 
> Shall we queue it up for 3.2? It's a good way to beat on fs discard
> support, fio could be easily configured for that.
> 

That would be great, thanks!

Btw I am not sure what do you mean by "beat on fs discard support" :).

-Lukas
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