XFS fragmentation on file append

[Keyur Govande]

On Thu, Apr 24, 2014 at 2:54 AM, Stefan Ring <stefanrin at gmail.com> wrote:
> I've become interested in this topic, as I'm also running MySQL with
> O_DIRECT and innodb_file_per_table. Out of curiosity, I immediately
> ran xfs_bmap on a moderately sized table space (34GB). It listed
> around 30000 fragments, on average one for every MB.
>
> I want to report what happened then: A flurry of activity started on
> both disks (root/swap lives on one of them, the data volume containing
> the MySQL files on another) and lasted for about two minutes.
> Afterwards, all memory previously allocated to the file cache has
> become free, and also everything XFS seems to keep buffered internally
> (I think it's called SReclaimable) was released. Swap usage increased
> only slightly. dmesg was silent during that time.
>
> This is a 2.6.32-358.2.1.el6.x86_64 kernel with xfsprogs 3.1.1 (CentOS
> 6.4). The machine has 64GB of RAM (2 NUMA nodes) and 24 (virtual)
> cores. Is this known behavior of xfs_bmap?

Interesting...it looks like your box flushed all of the OS buffer
cache. I am unable to reproduce this behavior on my test box with the
3.10.37 kernel. I also tried with 2.6.32-358.18.1.el6.x86_64 and
didn't hit the issue, but obviously our access patterns differ wildly.