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RAID 4 - Data Striping With a Dedicated Parity Drive
RAID 4 works in the same way as RAID 0. The data are striped amongst the hard disks. Ad-
ditionally, the controller calculates redundancy data (parity information) which are stored
on a separate hard disk (P1, P2, ...). Even when one hard disk fails, all data are still fully
available. The missing data is recalculated from the data still available and the parity in-
formation. Unlike in RAID 1, only the capacity of one hard disk is needed for the redun-
dancy. If we consider, for example, a RAID 4 disk array with 5 hard disks, 80% of the
installed hard disk capacity is available as user capacity, only 20% is used for redundancy.
In situations with many small data blocks, the parity hard disk becomes a throughput bot-
tle-neck. With large data blocks, RAID 4 shows significantly improved performance.
RAID 5 - Data Striping with Striped Parity
Unlike RAID 4, the parity data in a RAID 5 disk array are striped in all hard disks. The RAID 5
disk array delivers a balanced throughput. Even with small data blocks, which are very likely
in a multi-tasking and multi-user environment, the response time is very good. RAID 5 of-
fers the same level of security as RAID 4. When one hard disk fails, all data are still fully
available, the missing data are recalculated from the data still available and the parity in-
formation. RAID 4 and RAID 5 are particularly suitable for systems with medium to large
capacity requirements, due to their efficient ratio of the installed and actually available ca-
pacity.