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.
My questions are: Is there a reason for this difference, and is one of
the answers more correct than the other?
Rich.
[1] https://www.redhat.com/archives/libguestfs/2018-January/msg00002.html
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines. Supports shell scripting,
bindings from many languages. http://libguestfs.org
--
To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
