The following is from Delltechcenter’s forum, posted by Joe from Dell EQL regarding the confusions about Equallogic’s load balancing between pools with multiple array members.

The following best practices should be considered for storage pools:

1. Do not mix arrays with different drive speeds RPM within a single pool unless they are running a unique RAID policy

2. Do not mix arrays with different drive technologies (SATA, SAS, SSD) within a single pool unless they are running a unique RAID policy.

3. Do not mix arrays with different controller speeds (1GbE, 10GbE) within a single pool unless they are each running unique RAID policies.

4. To override the automated performance decisions for a specific volume, indicate a ―preferred RAID type for that volume. If that RAID type exists within the pool, the volume will reside on those arrays that match the preferred RAID policy for the volume.

For example: if you have two members with different drive technologies (Item #2), so you can create each member with a different RAID preference (as you indicated Array1 w/Raid50, Array2 w/RAID5), and then specify the preferred RAID policy for the volume(s), without any issues.

With volume RAID preferences set to Auto, the volume will span both members in the pool. If using the RAID preference, it isn’t a guarantee that it will adhere to the request and only place the volume on a given member; for instance if the volume is too large to fit on the member with the desired RAID level (you might end up with an 80/20 balance).

Note that when mixing SAS/SATA in the same pool the performance load balancer doesn’t take drive RPM into account in its calculations. A common issue is that if you combine fast SAS with slower SATA (often worst with the SATA setup as RAID 5) the SAS array won’t provide its potential performance, and the load balance may place more data on the larger SATA member (typically SATA Arrays have more space).

In other words, old RAID theory still applies here: DO NOT MIX different RPM disks in a raid group.