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

[Hannes Reinecke <hare@suse.de>]

On 08/07/2015 09:44 AM, Ming Lei wrote:
> On Fri, Aug 7, 2015 at 3:33 AM, Christoph Hellwig <hch@lst.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@suse.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.
> 
Which is why I made it optional, and having to use some ioctl fields
to enable this feature.
So _if_ someone uses these new features _and_ then complains that
the sector size is different I'll have only limited compassion.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
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