On Mon, Jan 08, 2018 at 10:43:05PM +1100, Dave Chinner wrote: > 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 That makes a lot of sense, thanks. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top -- 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
