Upgrading a PERC h310 to a PERC H710 mini RAID controller on a Dell R620

The Dell PERC H310 controller does not have the ability to use write cache.

H310 — Entry hardware RAID controller. Provides entry-level performance with no cache. RAID5 functionality with modest performance.

Your write performance will suffer as a result of this. This is a very bad controller for ESXi or virtualization using local disks.

Note that with a RAID 5 implementation, you will see immense penalty for small writes. Any write which is not going to overwrite an entire stripe (i.e. is smaller than the stripe set size) will induce a read operation off all disks, the re-calculation of the XOR parity and a write operation to at least two of the array's disks. Using a 5 disk array and a 64KB stripe element size (710's default and minimum), this would happen for every write operation smaller than 256KB. The basic outcome is that for a small writes workload, your write performance is going to be even worse than a single drive of your array would be capable of performing.

This obviously would cause everybody drop the idea of using RAID5 ever if it were not for the caches and the splendid read performance a RAID5 array is exhibiting. The caches would usually catch any small writes, possibly group them and flush them to disks whenever convenient. If small writes only happen occasionally and do not overrun the cache buffers, you are not likely to see problems with RAID5 performance. If the cache buffers are already filled with yet unflushed requests or if the cache is not set up to work as "write-back" (or has a non-functional / discharged BBU), you will see abysmal performance from the array. So you should be checking both - your access pattern as well as the cache status of your controller.