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

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Message-ID: <CACVXFVNeWJ1W=n1EY3Ffx_2Ft+uy7zVXi_ZhZHfPwkE7k1zDnA@mail.gmail.com>
Date:	Fri, 7 Aug 2015 04:02:18 -0400
From:	Ming Lei <tom.leiming@...il.com>
To:	Hannes Reinecke <hare@...e.de>
Cc:	Alexander Graf <agraf@...e.de>, Jens Axboe <axboe@...com>,
	Christoph Hellwig <hch@....de>,
	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 3:45 AM, Hannes Reinecke <hare@...e.de> wrote:
> On 08/07/2015 09:23 AM, Ming Lei wrote:
>> On Fri, Aug 7, 2015 at 2:46 AM, Hannes Reinecke <hare@...e.de> wrote:
>>> On 08/07/2015 07:07 AM, Ming Lei wrote:
>>>> On Mon, Aug 3, 2015 at 7:00 PM, Alexander Graf <agraf@...e.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.

OK, sounds a valid case, and suggest to add the install bootloader story
into the commit log.


thanks,
Ming Lei
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