XX102-00-00 Rev 203 KOL-RAID3 RAID Storage Systemd
Appendix B: Technology Background
•
51
Due to the data redundancy of mirroring, the drive capacity of the array is only the size of the smallest drive. For example,
two 100GB drives which have a combined capacity of 200GB instead would have 100GB of usable storage when set up in
a mirrored array. Similar to RAID 0 striping, if drives of different capacities are used, there will also be unused capacity on
the larger drive.
RAID 0+1: Striping/Mirroring
Striping/mirroring combines both of the previous array types. It can increase performance by reading and writing data in
parallel while protecting data with duplication. At least four drives are needed for RAID 0+1 to be installed. With a four-
drive disk array, drive pairs are striped together with one pair mirroring the first pair. The data capacity is similar to a
standard mirroring array, with half of the total storage capacity dedicated for redundancy. An added plus for using RAID
0+1 is that, in many situations, such an array offers double fault tolerance. Double fault tolerance may allow your data
array to continue to operate depending on which two drives fail.
2
4
6
8
1
3
5
7
2
4
6
8
1
3
5
7
Disk Drives
Data
Mirror
Data Stripe
Figure 17. RAID 0+1 Striping and Mirroring of Two Drive Pairs
RAID 3: Block Striping with Parity Drive
RAID level 3 organizes data across the physical drives of the array, and stores parity information on to a drive dedicated
to this purpose. This organization allows increased performance by accessing multiple drives simultaneously for each
operation, as well as fault tolerance by providing parity data. In the event of a hard drive failure, data can be re-calculated
by the RAID system based on the remaining drive data and the parity information.
The adjustable block size of the RAID 3 array allows for performance tuning based on the typical I/O request sizes for
your system. The block size must be set at the time it is created and cannot be adjusted dynamically. Generally, RAID
Level 3 tends to exhibit lower random write performance due to the heavy workload going to the dedicated parity drive for
parity recalculation for each I/O.
The capacity of a RAID 3 array is the smallest drive size multiplied by the number of drives less one. Hence, a RAID 3
array with (4) 100 GB hard drives will have a capacity of 300GB. An array with (2) 120GB hard drives and (1) 100GB hard
drive will have a capacity of 200GB.
Data Drives
Parity Drive
1 parity
2 parity
3 parity
4 parity
1d
2d
3d
4d
1c
2c
3c
4c
1b
2b
3b
4b
1a
2a
3a
4a
Data Stripes
Figure 18. RAID 3 Multiple Drives Stripe Data w/ One Dedicated Parity Drive