I am planning the configuration for my filers. I am doing some research on the recommentation on the root volume sizing. I have 2 approaches now:
1. Create a large 14 disks raid-dp aggregate to host both the root volume and other application volumes.
2. Create a 2 disks raid 4 aggregate to just host the root volume. Create another aggregate for application volumes.
# 2 sounds reasonable as I heard there is administration advantages on separating the root volumes and the application volumes into different aggregate. But at the same time, I find that there is paper (http://www.peripheralstorage.com/NetApp/pdfs/SynchronousSnapMirror.pdf) saying that it's not good idea to configured root aggregate as raid 4 since the number of disk can't provide adequate write performance on snapmirror. Any thoughts?
The issue with a 2 drive root aggregate (raid4) is that you don't get hot upgrades of disk firmware since that requires dual parity. You could add a disk, hot upgrade, remove the dual parity and zero it, but that is a lot of work every time you upgrade firmware.
The recommendation for more than 3 drives (7 drives) for snapmirror is when we are using snapmirror sync/semi-sync where the cplogs and nvlogs get written to the root aggregate and more spindle I/O is needed. It isn't a good idea to run synchronous snapmirror with a small root aggregate. With async snapmirror this is not an issue.
Per the above link you will find detailed discussion and debate on a separate or combined root aggregate. I tend to create separate but when a smaller system I agree it is a big cost not worth the rare risk of aggr failure. However, note the 8.0 64-bit discussion. A root aggregate must be 32-bit. So, if you want all 64-bit large aggregates in 8.0, you will need a separate 32-bit root aggregate.
The recommendation for more than 3 drives (7 drives) for snapmirror is when we are using snapmirror sync/semi-sync where the cplogs and nvlogs get written to the root aggregate and more spindle I/O is needed.
In current DataONTAP logs a written to aggregate that hosts target volume.