HP D5970A - NetServer - LCII Configuration Manual page 18

Chapter 2
RAID 3: Striping with Dedicated Parity
In RAID 3 configurations, each data stripe generates one parity block to provide
redundancy and data protection. The parity block is encoded information that can
be used to reconstruct the data on that stripe if one of the disks fails. RAID 3
configurations dedicate one disk in the array to store all parity blocks. If you have
five physical drives configured as one RAID 3 logical drive, data blocks are
written as follows:
Disk 1
Stripe 1
Block 1
Stripe 2 Block 5
Stripe 3 Block 9
With RAID 3, data reads are faster than writes, because parity must be calculated
for writes. RAID 3 performs better for long writes than for short ones, because
writes of less than one full stripe involve a parity calculation. RAID 3 works well
for long data transfers, such as CAD files and data logging.
RAID 3 Advantages
There is no data loss or system interruption due to disk failure, because if one
disk fails, data can be rebuilt.
Only one disk in the RAID 3 logical drive is reserved to provide redundancy.
HP NetRAID firmware optimizes RAID 3 data flow for long, serial data transfers
such as video or imaging applications.
RAID 3 Disadvantages
Performance is slower than RAID 0 or RAID 1.
RAID 3 Summary
Choose RAID 3 if cost, availability, and performance are equally important.
RAID 3 performs best when long, serial transfers account for most of the reads
and writes.
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Disk 2 Disk 3
Block 2 Block 3
Block 6 Block 7
Block 10 Block 11
RAID Overview
Disk 4 Disk 5
Block 4 Parity 1-4
Block 8
Parity 5-8
Block 12
Parity 9-12