How to add a new raid group to an existing aggregate

When adding disks to a NetApp system, you can create a new aggregate, or add disks to an existing aggregate.  Depending on your use case, you can choose which option is best.  However, to add an disks to an existing aggregate, do the following:

aggr add <aggr-name> -g new -d <disk-list>

You will be prompted to proceed, cancel, or preview the new RAID layout.  You should preview the layout to verify it is what you wanted.

Note: preparing to add 21 data disks and 2 parity disks.
Continue? ([y]es, [n]o, or [p]review RAID layout)

As best practices dictate, the new raid-group size should be the same as the existing disks.

For example, if you have a 23 drive aggregate, you'll want to add 23 more disks to a new raid group in the same aggregate:

aggr add <aggr-name> -g new -d 4d.13.0 4d.13.1 4d.13.2 4d.13.3 4d.13.4 4d.13.5 4d.13.6 4d.13.7 4d.13.8 4d.13.9 4d.13.10 4d.13.11 4d.13.12 4d.13.13 4d.13.14 4d.13.15 4d.13.16 4d.13.17 4d.13.18 4d.13.19 4d.13.20 4d.13.21 4d.13.22

You will get a message:

23 disks need to be zeroed before they can be added to the aggregate. The process has been initiated. Once zeroing completes on these disks, all disks will be added at once and you will be notified via the system log. Note however, that if system reboots before the disk zeroing is complete, disks will not be added.

No big deal, just wait.  Depending upon the size of the disks, it will can take a long time before they are added.

Now, don't forget to reallocate all the volumes after the aggregate is expanded.  NetApp doesn't re-stripe all the disks when adding storage to an aggregate - it just slaps the new raid group next to the old one.  If you do not reallocate, the new writes will end up on the new disks, and you'll not be able to take advantage of all disks working in unison.  

If you add a raid group of not the same size, you can get severely imbalanced I/O patterns.  One drive off won't be too bad, but I've seen aggregates with a 16 and a 3 drive raid group.  The 4 drive raid group gets hit much harder, leading to hot spots.  It's very important to build your raid group sizes identically.