Use RAID 1 for metadata LUNs and RAID 5 for data LUNs.
Use RAID 1 for metadata LUNs
RAID 1 (mirroring) can give slightly better performance than the default RAID 5 scheme for the small, two-drive metadata LUNs that Xsan uses to store volume information. A single drive is almost always adequate for storing the primary volume metadata (10 GB of metadata space is enough for approximately 10 million files.) The second, mirror drive protects you from metadata loss.
Use RAID 5 for data LUNs
Most RAID systems are optimized for excellent performance and data redundancy using a RAID 5 scheme. (RAID 5 stripes data across available drives and distributes parity data across the drives.) Some RAID systems ship preconfigured as RAID 5 LUNs. RAID 0 (striping with no parity) might give slightly better write performance, but it provides no data recovery protection, so RAID 5 is always a better choice for LUNs used to store user data.