[CentOS] Expanding RAID 10 array, WAS: 40TB File System Recommendations

[Emmanuel Noobadmin]

On 4/14/11, Ross Walker <rswwalker at gmail.com> wrote:
> You can't expand a mdraid raid0.
>
> I believe you can expand a mdraid raid10,5,6, but not raid0.

That was what I thought previously when looking into this and weighing
the pros/cons of using RAID 10 vs RAID 5.

But earlier this week, from the 40TB Filesystem thread, Rudi stated
that he has a RAID 0 on RAID 1 setup that he can expand and Brandon
confirmed that it is possible to expand RAID 0.

http://forum.nginx.org/read.php?24,190407,190764#msg-190764

Since I've read that some features such as creating RAID 10 directly
were not in the man pages, and another person in the mdadm list
implied that a raid 10 like array could be achieved using some
creative RAID 5 layout, I assumed that perhaps it was possible to do
that, just that there was some undocumented trick or specific manner
the arrays had to be setup.

I'm was stuck trying to decide whether to go for the cheaper RAID 5
setup and possibly getting killed by the IOPS penalty and the risk
associated with rebuild time, or figure out a way to use the
recommended RAID 10 setup with a smaller usable capacity for the
budget but do so with the ability to expand in the near future. So
really hoping that it could be done.


> I often find it handy to have a "backup" raid1 disk on the system
> that's big enough to hold the contents of the largest LV, then dump
> the production LV to the backup, blow away the production, recreate
> with the new stripe size, then restore the data back. This backup
> volume could be an iSCSI volume exported from another server that does
> have the capacity if there isn't any in the host.

That sounds like a solution since each LV shouldn't be too massive.
This seems to imply I would have to bring down the service running off
the LV during the recreation but I suppose I since the storage is
planned to be exported over iSCSI in the first place, I could simply
export the backup copy while expanding the original.