Re: statfs b_avail & b_free different if the filesystem is mounted readonly

[Dave Chinner <david@fromorbit.com>]

On Mon, Jan 08, 2018 at 08:44:50AM +0000, Richard W.M. Jones wrote:
> We had a question[1] posed by a libguestfs user who wondered why the
> output of ‘virt-df’ and ‘df’ differ for an XFS filesystem.  After
> looking into the details it turns out that the statfs(2) system call
> gives slightly different answers if the filesystem is mounted
> read-write vs read-only.
> 
>   ><rescue> mount /dev/sda1 /sysroot
>   ><rescue> stat -f /sysroot
>     File: "/sysroot"
>       ID: 80100000000 Namelen: 255     Type: xfs
>   Block size: 4096       Fundamental block size: 4096
>   Blocks: Total: 24713      Free: 23347      Available: 23347
>   Inodes: Total: 51136      Free: 51133
> 
> vs:
> 
>   ><rescue> mount -o ro /dev/sda1 /sysroot
>   ><rescue> stat -f /sysroot
>     File: "/sysroot"
>       ID: 80100000000 Namelen: 255     Type: xfs
>   Block size: 4096       Fundamental block size: 4096
>   Blocks: Total: 24713      Free: 24653      Available: 24653
>   Inodes: Total: 51136      Free: 51133
> 
> ‘virt-df’ uses ‘-o ro’ and in the ‘df’ case the user had the
> filesystem mounted read-write, hence different results.
> 
> I looked into the kernel code and it's all pretty complicated.  I
> couldn't see exactly where this difference could come from.

Pretty simple when you know what to look for :P

This is off the top of my head, but the difference is mostly going
to be the ENOSPC reserve pool (xfs_reserve_blocks(), IIRC). it's
size is min(%5 total, 8192) blocks, and it's not reserved on a
read-only mount because it's only required for certain modifications
at ENOSPC that can't be reserved ahead of time (e.g. btree blocks
for an extent split during unwritten extent conversion at ENOSPC).

The numbers above will be slightly more than 5%, because total
blocks reported in fsstat doesn't include things like th space used
by the journal, whereas the reserve pool sizing just works from raw
sizes in the on-disk superblock.

So total fs size is at least 24713 blocks. 5% of that is 1235.6
blocks. The difference in free blocks is 24653 - 23347 = 1306
blocks. It's right in the ballpark I'd expect....

> My questions are: Is there a reason for this difference, and is one of
> the answers more correct than the other?

Yes, there's a reason. No, both are correct. :P

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com