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From: Mike Fleetwood <mike.fleetwood@googlemail.com>
Date: Sat, 19 May 2018 09:36:54 +0100
Message-ID: <CAMU1PDgUFvA6qTbMoWjPck_oN2EjH6C_Jy63suTgqpnXCzpw6w@mail.gmail.com>
Subject: Re: How to reliably measure fs usage with reflinks enabled?
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To: Eric Sandeen <sandeen@sandeen.net>
Cc: Dave Chinner <david@fromorbit.com>, Tarik Ceylan <Tarik.Ceylan@ruhr-uni-bochum.de>, linux-xfs@vger.kernel.org

On 18 May 2018 at 15:56, Eric Sandeen <sandeen@sandeen.net> wrote:
> On 5/18/18 9:43 AM, Mike Fleetwood wrote:
>>
>> (Sorry for the late reply, work commitments)
>>
> ...
>
>> So after checking, GParted was modified to use the dumpe2fs command to
>> read the superblock to get the file system size for mounted ext* file
>> systems too.
>>
>> https://marc.info/?l=linux-ext4&m=134706477618732&w=2
>>
>> I see that xfs_db doesn't allow reading the super block of mounted XFS
>> file systems.  So for the case of a mounted XFS on full fat block device
>> I guess I'll wait and see how much overhead is subtracted from the
>> statvfs f_blocks figure and make sure GParted accounts for that.
>
>
> Actually you can, with -r:
>
> # mount /dev/sda1 /mnt/test
> # xfs_db -r -c "sb 0" -c "p dblocks" /dev/sda1
> dblocks = 229771264
>
> though I may be giving you rope to hang yourself here ;)
>
> It's generally a bit dicey to be reading a mounted block device for any
> filesystem, as there's no coordination with changes the filesystem may
> be making while it's mounted.
>
> The XFS_IOC_FSGEOMETRY would be a better choice for gathering geometry
> information for a mounted xfs filesystem.

Thanks, the xfs_db -r option is exactly what I need.  (Should have
checked the man page myself for that).

Mike