Raid 3; Raid 5 - Acer Altos RAIDWatch Manual

Altos RAIDWatch User Guide
C.6 RAID 3
RAID 3 implements block striping with dedicated parity. This RAID level breaks data into logical
blocks, the size of a SCSI disk block, and then stripes these blocks across several drives. One drive is
dedicated to parity. In the event a disk fails, the original data can be reconstructed from the parity
information.
In RAID 3, the total disk capacity is equivalent to the sum of the capacities of all SCSI drives in the
combination, excluding the parity drive. Thus, combining four 250GB SATA drives, for example,
would create a single logical drive with a total disk capacity of 750GB. This combination appears to
the system as a single logical drive.
RAID 3 provides increased data transfer rates when data is being accessed in large chunks or
sequentially.
However, in write operations that do not span multiple drives, performance is reduced since the
information stored in the parity drive needs to be re-calculated and re-written every time new data
is written to any of the data disks.
C.7 RAID 5
RAID 5 implements multiple-block striping with distributed parity. This RAID level offers the same
redundancy available in RAID 3; though the parity information this time is distributed across all
disks in the array. Data and relative parity are never stored on the same disk. In the event a disk
fails, original data can be reconstructed using the available parity information.
For small I/Os, as few as one disk may be activated for improved access speed.
RAID 5 offers both increased data transfer rates when data is being accessed in large chunks or
sequentially and reduced total effective data access time for multiple concurrent I/Os that do not
span multiple drives.
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Block 1
Block 2
Block 3 Block 1 Block 2
Block 3 Block 4
Block 4
Block 5 Block 6
Block 5
Block 7 Block 8
Block 6
Block 7
Block 8
Parity (1,2)
Parity (3,4)
Parity (5,6)
Parity (7,8)