Much the way the many drives used in larger RAID arrays can tax the available space in a regular PC case, they do
the same to the capabilities of the system power supply. If you examine the output ratings of a typical power
supply, you'll find that the amount of +12 V power provided is more than adequate for as
many as four hard disks, but some lower-end supplies may not be able to handle more than
two. Virtually no regular PC power supply will be up to the challenge of providing
adequate power to a RAID array of 8, 10, 12 or more hard drives. This is particularly true
when you consider the peak startup draw
of hard drive motors.
For this reason, it's important to carefully check the capabilities of the power supply
when implementing a larger RAID array. The need for power is another reason why larger
RAID arrays are usually implemented in either specialized server cases or external enclosures. These cases are matched with
larger power supplies that can handle the load of many hard drives. Often these cases will
in fact be equipped with redundant
power supplies, which provides fault protection against problems with the power
supply. If one supply fails the other will continue to seamlessly provide power to the
entire array. I recommend these for those who are setting up a RAID array for fault
tolerance and can afford them.
Tip: Power supplies
provide power using the integrated power cables and connectors that come from the power
supply box. You will often notice that these connectors are in separate
"groups"; for example, there may be 8 power connectors arranged into two groups
of four connectors. If at all possible, "spread these around" so that if a fault
develops with one set of cables it will not take the entire array down. In the example
above, if you are running a RAID 1+0 array, "split" each of the mirrored pairs
so one drive is attached to one group and the other drive is attached to the second group.
Even if the power is cut to entire group then, the array will stay up since one drive in
each RAID 1 sub-array will still be powered.
Another issue with larger arrays is cabling: it can become quite a mess if you have a
large number of drives. Snaking four large SCSI cables to a dozen hard drives and running
the power cables to them isn't a lot of fun. Separate cables also make drive swapping difficult to impossible, so a drive failure
means taking down the system. Larger cases will help to keep the cabling from becoming
unmanageable, but a better solution now being used by many server cases is a SCSI
variation called single connector attachment or
SCA.
In an SCA system the separate data and power cables that normally run to each drive are
eliminated and replaced with a single 80-pin connector. Special backplanes are
installed in the server case and the drives snap into the mating connectors on the
backplanes, in a process not dissimilar to how you connect a printer cable to a parallel
port. The connection between the backplanes and the RAID controller is greatly simplified
compared to running data and power cables to each drive, and SCA is designed specifically
to allow drive swapping. Most RAID enclosures and server cases are now designed to use SCA
due to its significant advantages for RAID arrays.
A final not relates to controllers that use multiple
channels. If your controller has this useful feature then you should always use all of
the channels available to improve performance. The optimal way of connecting the drives
depends on the number of channels you have and the type of RAID array you have
implemented. In general, you want to evenly distribute the drives between the channels to
allow as little contention as possible within each channel, and to improve throughput. So
if you have a six-drive array on a three-channel controller, put two drives on each
channel. If you have eight drives, split them up as into groups of 2, 3, and 3.
Next: RAID
Management
