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

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Date:	Thu, 18 Aug 2011 21:24:35 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Lukas Czerner <lczerner@...hat.com>
CC:	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 2011-08-18 21:20, Lukas Czerner wrote:
> 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!

Alright, lets do that.

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

Perhaps worded a bit weird, what I mean was "thoroughly exercise and
test file system discard support" :-)

-- 
Jens Axboe

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