RAID-ing two drives...why would you?

gnatsum · Sep 2nd, 2005, 03:10 PM

Other than the obvious double the space on a simulated single drive, why else would you want to raid two drives together?

MacDoc · Sep 2nd, 2005, 03:15 PM

Cuz it's twice as fast 111 megs per second versus 55

elmer · Sep 2nd, 2005, 03:16 PM

	Quote:
	Other than the obvious double the space on a simulated single drive,

Another concept to be aware of is that both drives can be accessed simultaneously, in parallel. This may improve performance, depending on a few factors.

Also, some types of RAID use it for redundancy, so that even if one drive failed you could keep working seamlessly. Once a replacement drive is hot-plugged in it would be automatically regenerated from the data on the other drives, after which your redundancy would be in place again.

gnatsum · Sep 2nd, 2005, 03:48 PM

really...

so, what if the info which is stored that i need happens to be on the sector which is on the the failed drive, it wont be destroyed?

also, can i raid a partition of a drive, to another drive?

and does that mean that two 80 gig drives will serve me better than 1 160 gig drive?

MacDoc · Sep 2nd, 2005, 03:51 PM

You cannot partition a RAID set at the moment

You need a secondary backup.

If you MIRROR you don't get the speed or the space - you do get perhaps overkill security.

I assume you are talking G5 tower.
A pair of 80s in Level 0 will be twice the speed of a single 160 drive.

CanadaRAM · Sep 2nd, 2005, 04:12 PM

	Quote:			Originally Posted by MacDoc
	Cuz it's twice as fast 111 megs per second versus 55

Not true.
See www.storagereview.com for lots of detail about why, in desktop single user usage patterns, a RAID 0 is no faster and can actually be slower than a single drive. In addition, you are constrained by both the drive controller and the RAID software. Two drives on a single ATA or SATA or Firewire controller are limited by the throughput of the controller and the buss. The software RAID used by Apple robs time from your CPU to calculate the distribution of data across the disks. If you had a RAID set up with hardware RAID controllers, and a separate buss for each drive, ($$$$) you would still not approach the 2x theoretical speed because of controller overhead and PCI buss limitations.

RAID - Redundant Array of Inexpensive Disks

Redundancy (or mirroring) means that the data you write to the hard drives is stored with a backup so that if you suffer the loss of one drive in the array, you can reconstruct all of your data from the remaining drives.

Array means more than one drive working together and appearing to your machine as one storage device

Inexpensive related originally the difference in price between a group of 3 to 10 small hard drives and the expense of a single gigantic drive used in servers. This distinction is largely academic now.

The RAID function is controlled either by software or by a dedicated RAID controller card. There are two components of a RAID array; striping and redundancy (or mirroring).

Striping takes your data and divides the bytes across two or more hard drives. The theory is that the writing and reading speed will be faster because as drive A is dealing with one byte, drive B is already getting a head start finding the next byte to read or write. This works because transferring the data takes only 20% of the time needed, the other 80% of the time the drive needs to move the heads and wait for the data on the disk to rotate around to the head.

The simplest RAIDs are called RAID 0 (striping) and RAID 1 (mirroring)

RAID 0 simply uses two identical hard drives, and stripes the data between them to create a single volume. There is no redundancy, therefore if one drive has a failure, ALL the data is lost on both drives. Cheapest but risky. 2 x 100 Gb drives in RAID 0 = 200 Gb space

RAID 1 just means two drives, where data is written simultaneously to both. Drive B is a mirror of drive A. This provides good redunancy, at the expense of half or the drive space. 2 x 100 Gb in RAID 1 = 100 Mb space.

RAID 2 through 5 are various arrangements of 3 to 10 drives, where data is striped across the drives, but "parity" data is also stored, to be able to reconstruct any single drive. Typically one drive is consumed by the parity data, so
3 x 100 Gb drives in RAID = 200 Gb space
5 x 100 Gb drives in RAID = 400 Gb space

You can also combine RAIDs, such as a striped pair (RAID 0) mirrored by another identical striped pair (RAID 1), which is sometimes called RAID10 or RAID 1+0 or RAID 0+1
4 x 100 Gb drives in RAID 0+1 - 200 Gb space.

Bottom line:
For desktop computer use, RAIDs are almost never worth it, unless you are doing something that needs massive data streaming, like video production.

There is an overhead to RAID in calculating where each byte of data is going to go. This means that RAIDs are slower than a single drive for everyday use.

They only really shine when put under the loads of a server, or if you need a single volume that exceeds the size of available single drives.

RAID 0 is terribly risky unless you maintain a fulltime backup onto another drive. RAID 1 - 5 provide additional security but have significant costs for drives, controller cards, and drive enclosures. But remember that RAID 1 - 5 only protect against drive failure. If you delete a file, have a crash or get a virus, the damage is done instantaneously to the mirror as well as the primary data, so RAID gives you no protection against that.

Thanks
Trevor
CanadaRAM.com

ArtistSeries · Sep 2nd, 2005, 04:16 PM

	Quote:			Originally Posted by CanadaRAM
	RAID 0 is terribly risky unless you maintain a fulltime backup onto another drive.

Excellent post Trevor.
Now you know why MacDoc is always reminding us to backup.

gnatsum · Sep 2nd, 2005, 04:23 PM

WOAH

okay, THAT explained EVERYTHING...i swear i can go teach a lesson on RAID now.

anyway, thanks a lot.

this thread is done.

If anyone has a question about raid-ing a drive ask Trevor.

He knows it all.

an no macdoc, it's no g5 tower. not by a longhorn, err. longshot.

lol i was just thinking because some people were advertising raid two drives which they are selling for twice the speed. so i was just curious.

because i like speed.

groovetube · Sep 2nd, 2005, 04:40 PM

if you like speed, raid. Screw redundancy.

that's what firewire+CCC is for.

elmer · Sep 2nd, 2005, 04:41 PM

Trevor makes lots of good points. Here are some more:

RAID is a really cool, geeky thing to do to your system.
Anything that cool tends to suck money out of your wallet spontaneously, even if it does have the word "inexpensive" in the acronym.
Some system architectures are just crying out for RAID - high bandwidth interface to the drive, large premium for high capacity drives, overall performance potential which can be unlocked with a little boost ... ie. G5 PowerMac.

	Quote:
	For desktop computer use, RAIDs are almost never worth it, unless you are doing something that needs massive data streaming, like video production.

To be fair, apparently Macdoc has a lot of customers who need to do exactly that. And if he says he's seen a doubling of performance, I believe him. Performance is subjective - a lot of reviewers forget that.