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

[Hannes Reinecke <hare@suse.de>]

On 08/07/2015 09:23 AM, Ming Lei wrote:
> On Fri, Aug 7, 2015 at 2:46 AM, Hannes Reinecke <hare@suse.de> wrote:
>> On 08/07/2015 07:07 AM, Ming Lei wrote:
>>> On Mon, Aug 3, 2015 at 7:00 PM, Alexander Graf <agraf@suse.de> wrote:
>>>>
>>
>> [ .. ]
>>
>>>>
>>>> because the guest thinks the disk is formatted with 4k sector size,
>>>> while mkfs thought it's formatted with 512 byte sector size.
>>>
>>> I am wondering if mkfs is remembering the sector size of actual block
>>> device, and at least it can't be found by 'dumpe2fs'. And it shouldn't have
>>> do that, otherwise it isn't flexible. And one fs image often can be looped
>>> successully by loop because loop's block size is 512.
>>>
>>> That is why I am wondering if we need support other logical block size
>>> for loop.
>>>
>> If you were to install a bootloader (like lilo or zipl for S/390) it
>> needs to write the _physical_ block addresses of the kernel and the
>> initrd. And these do vary, depending in the physical blocksize.
> 
> So there isn't filesystem involved in your case of installing bootloader,
> then I am wondering why you don't write the data to the backing block
> directly? And why does loop have to be involved in this special case?
> 
Because this is a virtual environment.
Hardware is a limited resource, and you would need to assign each
one to a guest.
Using loop you can run fully virtualized, without having to recurse
on hardware limitations.

>> So while the filesystems indeed do not care (all translation is done
>> in the block driver, not the filesystem), bootloaders most certainly
>> do.
>> If you were to create a bootable disk on 4k disks you need this patch.
> 
> It it were me, I choose to do that against the disk directly, instead of
> using loop, :-)
> 
See above. The reason why we did this patch is precisely because we
do _not_ want to use physical disks.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
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