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

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Date:	Tue, 4 Aug 2015 00:00:27 +0100
From:	Alexander Graf <agraf@...e.de>
To:	Ming Lei <tom.leiming@...il.com>, Hannes Reinecke <hare@...e.de>
Cc:	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 27.07.15 11:40, 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.

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

because the guest thinks the disk is formatted with 4k sector size,
while mkfs thought it's formatted with 512 byte sector size.

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.

Alex
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