A RAID is taking two or more disks and combining/coordinating them to work together in particular ways. "RAID" is an acronym for "Redundant Array of Inexpensive Disks".
There are basically two different types of RAID arrays. A "mirrored" RAID is where your data (and everything) is written on two disks in their entirety
and redundantly. As its name suggests, you get two disks that are exact copies of each other. The benefit of this type of RAID is that you get automatic, realtime up-to-the-second data backups. The downside is that, if you have say, two 180GB drives, your total disk capacity is NOT 360GB, but two identical 180GB drives. The speed of a mirrored array is the same as if you used just one of the disks as a normal drive.
The other main type of RAID is called a "striped RAID". This is where your data (and everything) is written ACROSS the two disks in a fragmented form and not in a redundant manner. No one disk of a striped array will have a complete writing of any file. Your file's content is distributed across BOTH drives when it is saved of copied to the striped RAID.
With a striped RAID, the RAID formatting software takes your two disks and essentially tells the OS to address them as ONE DISK. A striped array shows only ONE HARD DISK ICON on your desktop. Unlike a mirrored array, this ONE disk icon actually has the combined capacity of the two drives -- and almost the combined speed
... or at least a higher speed than if you were using a single disk scenario. The reason it is faster is because your data is saved across both drives at the same time, utilizing each drive's -- and the independent drive busses if your drives are connected in this manner -- at the same time. It's like being able to split into your two hands one big file folder of stuff and put them into two file cabinets at the same time with both hands. It's much faster and has your total disk capacity than a mirrored RAID offers. BUT
, if one of your striped disks goes down... ALL YOUR DATA IS LOST.
Although I have recently read of methods to possibly recover data from a stricken striped RAID, suffice it to say that a striped RAID is faster, bigger... but more risky. Using a striped RAID demands that you also have backup drives that you make regular backups to. I have a 360GB striped RAID, and 250GB of external Firewire drives to backup to. I know that math doesn't add up, but my backup drives are WAY below their capacity. With my entire RADI contents back-up'd I still have 123.63GB available on my FW drives.
If you can afford the drive space and have enough drive interfaces, you can also do two RAIDS: one that is striped for speed and another mirrored to back up the striped RAID. I don't know how such a complex array will affect system performance/bandwidth, however. Surely, there are others more experienced with RAIDs in this forum than I.
There are two types of RAID interfaces that you can use: a software RAID or a hardware RAID. A software RAID is where you can assign just about any two disks as your RAID and the RAID formatting software handles the formatting. The other type is by using a specially manufactured drive interface card designed for RAID operation. A software RAID is cheaper but a hardware RAID is faster -- the difference was about $100.00 in my case. It is best to have your two drives on TWO DIFFERENT drive interfaces or IDE channels (you can do SCSI and even -- I think -- Firewire RAIDS). Having the two drives on the same bus hinders the speed advantages of a RAID (including the mirrored one). So, I bought a Sonnet ATA133 card
. It does have dual ATA channels, and each of my to drives are on their own channel. The system is certainly faster than before, but it would be even faster had I purchased a dedicated RAID card
. I wasn't inclined to pay the difference at this time, however. There are also Serial ATA (or SATA) cards
available (SATA is what the G5 Macs use) -- and how about a striped RAID of EIGHT SATA DRIVES??!
Can you say, "ZOOM!!!"
The benefits of a striped array will show itself more obviously if you handle large files. Small files will benefit less from a striped RAID. Of course, if your current Mac has an ATA66 drive bus, 5200rpm hard drive with a 2MB cache, then ANY improvements will be helpful, RAID or not. This is what my Sawtooth had prior to the RAID (actually, the drives were 7200rpm and not the stock 5200rpm Sawtooth drives). My Xbench disk score before was 79.6 (29MB/second). With the two 7200rpm drives with 8Mb of cache I striped, my score is now 132.87 (45MB/second). Unfortunately, Xbench DOES NOT utilize a drive's RAM when testing, so both disks would actually be somewhat faster than this -- especially my new disks.
As for making a RAID? My limited experience so far on this:
I think matched drives are better (I think a mirrored RAID will only be as large as your smaller of the two drives). Matched drives may be required. I don't know.
Independent drive busses are most beneficial.
Dedicated RAID card is optimal.
Mac OS X's own Disk utility can configure software RAIDS and maybe even hardware RAIDS, although there are also third party RAID programs and the performance results of all these different formatting programs does vary. You'll have to do some further research to find all this out for sure.
Hope this helps! [img]smile.gif[/img]