linux-kernel - Re: [PATCH 0/4] loop: enable different physical blocksizes

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Date:	Fri, 7 Aug 2015 03:44:58 -0400
From:	Ming Lei <tom.leiming@...il.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Hannes Reinecke <hare@...e.de>, Jens Axboe <axboe@...com>,
	Alexander Graf <agraf@...e.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] loop: enable different physical blocksizes

On Fri, Aug 7, 2015 at 3:33 AM, Christoph Hellwig <hch@....de> wrote:
> On Mon, Jul 27, 2015 at 06:40:57AM -0400, Ming Lei wrote:
>> On Fri, Jul 17, 2015 at 3:27 AM, Hannes Reinecke <hare@...e.de> wrote:
>> > Currently the loop driver just simulates 512-byte blocks. When
>> > creating images for virtual machines it might be required to use
>> > a different physical blocksize (eg 4k for S/390 DASD).
>>
>> Looks 'qemu-img create' doesn't have parameter of block size,
>> so could you share your use case? And I am just curious why
>> 512-byte can't work for this case.
>
> The use case is to suppot 4k sectors sizes such as DASDs usually provide,
> or just to create a 4k block device to check your filesystem of choice
> handles 4k sectors just fine.  Replace 4k with other sector sizes of
> your choice for added benefit.  In addition to the DASD use case it's really a
> very useful debugging tool.

There shouldn't be any problem about looping over DASP which has
4k sector size. Also for debugging purpose, we can easily emulate 4k
sector size disk by QEMU/virtio-blk.

We can support 4k sector size on loop for debugging purpose too, but
the side effect is that some images can't be loop mounted any more
after its secror size is become larger, then people might complain that.

Thanks,
Ming Lei
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