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Answers
Well, with SAN it's different - and have in mind that not everything which pretends to be a SAN really IS a SAN ...
The problem with RAID10 is, that it is quite expensive, as you have to use the half amout of disks just for the mirroring. RAID5 does not mirror the data, but create Parity Sums (to restore the data if one disk is damaged). The writing of the Parity Sums is degrading the performance of write-transactions, thus not really recommended.
Real SAN (!) are using 20 spindles and more. It would cost a fortune if half of them would be just "wasted" for mirroring. Thus, SAN are usually running RAID5 variants (mostly RAID5DP or RAID6 or vRAID5):
The data is striped over e.g. 18 spindles, and 2 disks are used to write the double Parity Sums.
BUT: with real SAN this is NOT degrading performance, as all physical write transactions are perfromed asynchronously! The Fibre Channel headers provide a large piece of cache - NVRAM - usually 2GB and up. The SAN commits the write-transactions as soon as the data is in the cache - the SQL Server is virtually communicating with a sort of RAM disk. THEN the SAN is asynchronously transferring the data from the cache to the spindles (physical write), but writing the parities does not matter here.
So the short story is: with SAN - real SAN! - RAID levels using parity are not degrading perfromance, as a RAM cache is buffereing the data.
This also means, that Independent Power Supply and proper battery buffers are mandatory!
Many "wannabe SAN" just provide multiple spindles, but no or insufficient cache. With those storag systems a RAID5 & Co. will degrade performance of write-transactions.
As far as I know HP EVA is a "real SAN"
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