linux-kernel - Re: Bug in BLKBSZSET/GET ioctl ?

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Date:	Thu, 29 Dec 2011 12:20:10 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Wanlong Gao <gaowanlong@...fujitsu.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Bug in BLKBSZSET/GET ioctl ?

On Thu, Dec 29, 2011 at 11:51:45AM +0800, Wanlong Gao wrote:
> Hi all:
> 
> This is first reported to *libguestfs*: https://bugzilla.redhat.com/show_bug.cgi?id=624335
> 
> Then, I looked into upstream util-linux and it seems nothing wrong. I'm not convinced that it's a kernel bug.
> 
> produce:
> 
> ---
> # ./util-linux/disk-utils/blockdev --getbsz /dev/sda6
> 4096
> # ./util-linux/disk-utils/blockdev --setbsz 2048 /dev/sda6
> # ./util-linux/disk-utils/blockdev --getbsz /dev/sda6
> 4096
> # ./util-linux/disk-utils/blockdev --setbsz 512 /dev/sda6
> # ./util-linux/disk-utils/blockdev --getbsz /dev/sda6
> 4096

I think each blockdev invocation is working on a *new* bdev object.
You'll get consistent results if somehow keep it referenced, for
example, run a background copy task:

# cp /dev/sda6 /dev/null &

# ./util-linux/disk-utils/blockdev --getbsz /dev/sda6
# ./util-linux/disk-utils/blockdev --setbsz 512 /dev/sda6
# ./util-linux/disk-utils/blockdev --getbsz /dev/sda6

Thanks,
Fengguang
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