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E-6 Understanding Drive Arrays
Writer: Rickard Project: Compaq Smart Array 3200 Controller Reference Guide Comments: 340862-002
File Name: L-APPE.DOC Last Saved On: 12/7/98 9:30 AM
COMPAQ CONFIDENTIAL - NEED TO KNOW REQUIRED
Table E-1
RAID Level Characteristics
Distributed Data
Guarding (RAID 5)
Data Guarding
(RAID 4)
Mirroring
(RAID 1)
No Fault Tolerance
(RAID 0)
Usable Disk Space*
67% to 97% 67% to 97% 50% 100%
Disk Space Formula
(n = no. of drives)
(n-1)/n (n-1)/n n/2 n
Parity and
Data Redundancy
Parity distributed
over each drive
Dedicated parity drive Duplicate data None
Minimum Number
of Drives
3321
Comments
Tolerant of single
drive failures. Higher
performance than
RAID 4. Uses the
least amount of
storage capacity for
fault tolerance.
Tolerant of single
drive failures. Like
RAID 5, RAID 4 uses
the least amount of
storage capacity for
fault tolerance.
Tolerant of multiple,
simultaneous drive
failures. Higher
performance than
RAID 4 or 5. RAID 1
uses the most
storage capacity for
fault tolerance, and
requires an even
number of drives.
Best performance,
but data is lost if any
drive in the logical
drive fails. RAID 0
uses no storage
space for fault
tolerance.
*All drives are the same capacity.
If you require a fault-tolerant system for critical data, Compaq recommends
using RAID 5 for maximum storage space efficiency or RAID 1 if I/O
performance is more important.
If you store non-critical data, and space and performance are both important,
RAID 0 offers the best of both parameters. However, RAID 0 has no data
protection and you will have to rely on backups in the event of hardware failure.