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Subject: Re: [PATCH 0/4] loop: enable different physical blocksizes
From: Ming Lei <tom.leiming@gmail.com>
To: Alexander Graf <agraf@suse.de>
Cc: Hannes Reinecke <hare@suse.de>, Jens Axboe <axboe@fb.com>,
        Christoph Hellwig <hch@lst.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Mon, Aug 3, 2015 at 7:00 PM, Alexander Graf <agraf@suse.de> wrote:
>
>
> On 27.07.15 11:40, 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.
>
> If you only want to access the virtual disk inside of QEMU you're all
> good. However, if you want to directly run mkfs or fdasd on a loopback
> device, then you need to expose 4k blocksize to the tools or they end up
> creating a different on-disk format (if they work at all).
>
> So the easiest case where things go wrong is
>
>   $ qemu-img create test.img 1G
>   $ losetup /dev/loop0 test.img
>   $ mkfs.ext4 /dev/loop0

The default block size of mkfs.ext4 is 4K, so suppose it is 1024 by passing
'-b 1024'; otherwise inside VM, the block device(with 4k logical block size)
can be mounted correctly.

>   $ qemu <with lots of options to pass the disk as 4k disk>

Then you should pass 'logical_block_size=1024 or 512' in '-device '
parameter of qemu. The story is that block size of filesystem should
be equal or greater than logical block size of block device, see
sb_min_blocksize().

>
> 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.

>
> With this patch you can do
>
>   $ qemu-img create test.img 1G
>   $ losetup -B 4096 /dev/loop0 test.img
>   $ mkfs.ext4 /dev/loop0
>   $ qemu <with lots of options to pass the disk as 4k disk>
>
> and it will work because both views of the world are identical. The same
> applies for images you pull from a disk. So if you have a real 4k
> logical sector size disk and you dd an image from it, you won't be able
> to loop mount it today. With this patch set, you can.

No, the filesystem block size is just equal or bigger than logical block size
of the backing device, then it can be loop mounted successfully without
any problem.


Thanks,
Ming Lei
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