Accusys ExaSAN SW16 - Manual (Page 98)

RAID Overview
User Guide 5-2
synchronized; that is, anything written to one disk is also written to the other. Mirrored
data is very secure because if one disk fails, the data is available from the other disk.
5.2.3 Data Parity
The controller can generate “parity” for the ability to protect and rebuild data. Parity
protects stored information without requiring data mirroring. When data is protected by
parity, it is still available if a drive fails. Parity-protected data is reconstructed using the
parity formula. You can remove and replace a failed disk (known as “hot swapping”),
and the controller then rebuilds the data using the information on the remaining drives.
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.
5.3.1 RAID 0: Striping
RAID level 0, striping only, is the fastest and most efficient array type, but offer no fault-
tolerance. Any drive failure destroys the data in the array.
5.3.2 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.
5.3.3 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.
5.3.4 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.