Hardware Reference
In-Depth Information
overhead is still involved in the controller performing the striping, and issues still exist
with latency—that is, how long it takes to find the data—but performance will be higher
than any single drive can normally achieve.
Whensetupforreliability,arraysgenerally runRAIDLevel 1,whichissimple drivemir-
roring. All data written to one drive is written to the other. If one drive fails, the system
can continue to work on the other drive. Unfortunately, this does not increase perform-
ance, and it also means you get to use only half of the available drive capacity. In other
words, you must install two drives, but you get to use only one. (The other is the mirror.)
However, in an era of high capacities and low drive prices, this is not a significant issue.
Combining performance with fault tolerance requires using one of the other RAID levels,
such as RAID 5 or 10. For example, virtually all professional RAID controllers used in
network file servers are designed to use RAID Level 5. Controllers that implement RAID
Level5usedtobeveryexpensive,andRAID5requiresatleastthreedrivestobeconnec-
ted, whereas RAID 10 requires four drives.
With four 500GB drives in a RAID 5 configuration, you would have 1.5TB of total stor-
age, and you could withstand the failure of any single drive. After a drive failure, data
could still be read from and written to the array. However, read/write performance would
be exceptionally slow, and it would remain so until the drive was replaced and the array
wasrebuilt.Therebuildprocesscouldtakearelativelylongtime,soifanotherdrivefailed
before the rebuild completed, all data would be lost.
With four drives in a RAID 10 configuration, you would have only 1TB of total storage.
However, you could withstand many cases of multiple drive failures. In addition, after a
drive failure, data could still be read from and written to the array at full speed, with no
noticeable loss in performance. In addition, once the failed drive is replaced, the rebuild
processwouldgorelativelyquicklyascomparedtorebuildingaRAID5array.Becauseof
theadvantagesofRAID10,manyarerecommendingitasanalternativetoRAID5where
maximum redundancy and performance are required.
A typical low-cost SATA RAID controller enables up to four, six, or eight drives to be
attached, and you can run them in RAID Level 0, 1, 5, or 10 mode. PATA RAID cards
are available, but most RAID cards have moved to SATA, which doesn't have the mas-
ter/slave channel-sharing problems of PATA. SATA RAID cards use a separate SATA
data channel (cable) for each drive, allowing maximum performance. Motherboard-based
RAID controllers almost exclusively use SATA drives.
If you are considering a SATA RAID controller (or a motherboard with an integrated
SATA RAID controller), here are some things to look for:
