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MaxAttach NAS 6000 Administration Guide
RAID 5 Multiple Disk Striping with Distributed Parity
Description
RAID 5 is the default user data disk array configuration used in the NAS 6000 in arrays of
six disks and is probably the most popular RAID technique in the world. The factory
configured user drives in the Base Unit are configured as six disk RAID 5 arrays. RAID 5
in six disk arrays is also the recommend user configuration for Expansion Units.
This level is commonly referred to as striping with distributed parity. No single disk is
devoted to parity. The controller strips blocks of data across all drives in the array and then
the last data block is written as a parity check of the current write.
On a single drive failure, the parity and remaining data can be used to recreate the missing
drive data set. During the time of a drive failure, the array is in a "critical" state, where the
data is still safe and array operation continues. However, the failure of any additional drive
will cause the loss of all array data.
When a failed drive is replaced either manually or by a hot standby drive, the array rebuild
process starts, ultimately fully restoring the fault tolerant nature of the array. In the Maxtor
MaxAttach NAS 6000, a Hot Spare drive can be optionally configured that will
automatically take over for any failed drive.
A replacement disk or disks can be added to the array so that the system can carry on with
user access and full I/O operations even while the rebuild process takes place. The system
uses the parity information to recalculate the missing data elements. There is some
degradation of service during an online rebuild.
Fault Tolerance Cost
The fault tolerance cost is a function of the number of drives in the array where the
equivalent of one array drive's disk space is used for parity.
In RAID 0, there is no fault tolerance, and hence, no fault tolerance cost.
In RAID1, there is fault tolerance as 50% of the drives are used for data backup.
In RAID 5, the cost is approximately the equivalent of one drive per array. In a four drive
array, the cost is 25%. In a six disk array, the cost is 17%. In a ten disk array, the cost is 10%.
Chapter #11 - Appendix - Disk Array RAID Concepts
11/07/01 -- Revision 2.0.03A
Chapter #11 - Appendix - Disk Array RAID Concepts
RAID 5 Multiple Disk Striping with Distributed Parity
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