John,
Just a couple of remarks: I'm sure others will have had more practical experience.
So, as I (now) understand it, we've got to either:
- Override the 14 disk limit for a RAID group
I'm a little intrigued that you have it set to 14 (with the implication that you haven't had to increase it before). The default for new volumes has been 8 for some time now. Is this is a volume that was first created long ago, or are you (pardon the suggestion) still running an ancient ONTAP version?
The 2nd choice sounds like a Bad Idea in that according to this document, section 3.4: http://www.netapp.com/tech_library/3027.html, that the filer will try to distribute writes evenly between the old and new RAID group... if the new RAID group only contains two disks (we don't want to add more disk space than is needed now), this could be disatrous to performance
This is a real problem with growing a volume gradually: it's _never_ the right time to start a new RAID group :-(
Since 6.2(iirc) it's been possible to add new disks to any RAID group in a volume, not just the last one. So if you've had the foresight to have started with the right number of RAID groups, you can add disks to them round-robin. Not much help in your situation, though.
Is it possible to add a second parity disk to the same RAID group?
In 6.5, you can use DP (double parity), see the reference that Steve Strange posted here a couple of months ago:
http://www.netapp.com/tech_library/3298.html
At first glance, that seems to me like the least bad solution for you.
Chris Thompson Email: cet1@cam.ac.uk