Accusys ExaSAN SW-08 - Manual (Page 50)

5. RAID
User Guide Page 5-2
5.3 RAID Levels
The ExaSAN RAID system supports several RAID levels and configurations. Each level has a different
architecture and provides varying degrees of performance and fault tolerance. Each level has
characteristics to achieve maximum performance or redundancy depending on the data environment. See
Section 4.2.2.1, “Quick Setup,” for information about the available RAID configurations of your metadata
and data LUNs on your ExaSAN based shared storage system(s).
5.3.1 RAID 1: Mirroring
RAID level 1, mirroring, has been used for Metadata LUN because of its simplicity and high levels of
reliability and availability. Mirroring uses two drives, each drive stores identical data. RAID 1 provides
very high data reliability and improved performance for read-intensive applications, but this level has a
high capacity cost because it retains a full copy of your data on each drive in mirror set.
In a RAID 1 configuration, the capacity of the smallest drive is the maximum storage area.
5.3.2 RAID 5: Independent data disks with distributed parity
By distributing the parity information across all drives in a set, RAID level 5 achieves high reliability and
data availability. It also offers the highest read data transaction rate of all levels along with a medium
write rate. The low ratio of ECC (Error Correction Code) parity disks to data disks offers hardware
efficiency. Disk failure has a moderate impact on the total transfer rate.
5.3.3 RAID 6: Independent data disks with two Independent parity schemes
RAID level 6 extends RAID level 5 by adding an additional parity block; thus it uses block-level striping
with two parity blocks distributed across all member disks. RAID 6 does not have a performance penalty
for read operations, but it does have a performance penalty on write operations because of the overhead
associated with parity calculations.
RAID 6 is no more space inefficient than RAID 5 with a hot spare drive when used with a small number
of drives, but as arrays become bigger and have more drives the loss in storage capacity becomes less
important and the probability of data loss is greater. RAID 6 provides protection against data loss during
an array rebuild, when a second drive is lost, a bad block read is encountered, or when a human operator
accidentally removes and replaces the wrong disk drive when attempting to replace a failed drive.