How to use MacOS X's Built-in Software RAID?

[georgelien]

There is an option for RAID, I think, in Disk Utility.
Can someone teach me how to use it?
Thanks!

Regards,
George Lien
georgelien@email.com

[uoba]

This would be an interesting How To! Anyone?

[theed]

See also the "What is RAID" thread in this section. What is RAID?

Take 2 empty and exactly alike HD's, install, boot. Open Disk utility, and drag the drives (not the logical volumes) to a raid set of your choice. Boom. done.

I did mine with ATA, it may be possible but not advised to do so with removable drives. (firewire)

[georgelien]

Thank you.
But can you teach me how to mirror my current drive to another one?

[pianophile]

You should be able to simply install another disk of the same capacity (preferably not on the same IDE controller), boot up, and add the disk to your boot disk to form a RAID 1 mirror set as per the post above.

However, software RAID adds overhead, and I think it may disappoint you w.r.t speed. In general, hardware-based RAID is preferable. I suggest that you get a an Acard ATA133 IDE RAID card, two ATA133 drives, and mirror or stripe those. If you really want to get serious, get an external RAID 5 SCSI array.

[theed]

I don't mean to sound antagonistic, but I disagree completely w.r.t. speed of software RAID when using RAID 0 or 1. There is so little thinking that needs to occur that it's trivial to do the normal work in software.

As for simply mirroring a drive that already exists ... don't do that. Consider making a RAID array the same thing as formatting. (which it pretty much is) You can't format a disk and retain data. Just like you can't make a RAID set while retaining data. You'll have to move your data off of your drive to configure RAID on that drive. I don't think Apple's utility supports, or should support magical mirroring and RAIDifying a single drive onto another.

You could install the drive and just copy your data over. RAID -1 :-) I recommend that you never run you Hard Drive (main one anyway) at more than half full. If it's more than half full, you need a bigger drive. this keeps you from having to really care about disk and file fragmentation / optimization.

[btoneill]

Actually the point of RAID has nothing to do with speeding up the system. RAID, as the acronym stands for "Redundant Array of Independant Disks" (some also mention it being Inexpensive not Independent, but doesn't really matter). Now lets go over the different types of RAID, and what you gain by it, and what you lose. (I'll be skipping the hybrid/vendor-made-up RAID levels as it only gets more confusing )

RAID 0: This is just striping of data across disks. To be honest, this is not truely RAID, as no redundancy is added, but some reason this got the tag of RAID. What you do gain from RAID 0 is an increase in read and write speed due to the data going across different wires and being written to different platters. So instead of writing 100k on one platter on one disk, that 100k can be spread across 4 platters in 25k blocks. This allows you to do more reads and writes simultaneously.

RAID 1: This is mirroring of data. One disk is the exact copy of the other. You get both performance hits, and performance gains with this. The performance hit is you need to do a write twice the write (altho your write call returns after the first, the second is handled by the RAID software). The performance gain is being able to have two disks in which you can read the same data from.

RAID 2: It exists, but not even worth mentioning, it's purely academic

RAID 3:
RAID 4: These are basically the same, they create redundancy by creating a parity disk which is used to recreate the data of any disk that fails. Their differences have to do with the way the data is written to the non-parity disks, but not really relevant here. Both get very high read rates, but RAID 3 historically has very good write rates also, and RAID 4 has bad write rates, but depending on the implementation this may or may not be true (some hardware vendors have very fast RAID4)

RAID 5: This is probably the most common RAID, as it gets the best performance in relation to the cost. RAID 5 spreads the parity blocks across disks. In the event of a failure of any disk you have a X-1 in X chance of not having to recreate parity on a block (where X is the total number of disks in the RAID). You get an increase in read performance with RAID 5 and ok write performance.

Ofcourse, on all these it's assuming it's software RAID. The only one of these whose write rate will be faster then a single disk will be RAID 0, all the others will have a bottleneck of writes based on the sleep of the slowest disk. In hardware based RAID it's an entirely different ballgame, and you can take most of what you know about RAID performance and toss it out the window depending on the vendor.

As to the software RAID in OS X, it's not very useful for most folks. You do not have the ability to use RAID on your boot disk (unless you have an X Serve). So, if you wanted to mirror your drives you'd have to put in two additional drives for your data. As someone mentioned there is a card out ther now that claims they do RAID 1 and can boot OS X with it, but I've yet to see it, and can't see spending $150 to find out (it's done RAID 0 for awhile, but I could care less about RAID 0).

If you really want to learn more about RAID there is a decent tutorial at: http://www.ecs.umass.edu/ece/koren/a.../raidhome.html

Brian

[theed]

I hate it when people say that software RAID is worthless. It's a software implementation, like VPC is a software implementation of a PC. Sometimes it's exactly what you need, and free is a whole lot cheaper than $150.

I have 2 30G 5400 RPM drives combined in a stripe set. The throughput is roughly double what the individual drives are. The capacity is double what each drive is. Convenient for pushing around 20-40 G at a time.

As for compatibility with my OS, I'll take OS level software RAID over motherboard RAID any day.

And on the topic of performance, Hardware kicks everybody's butt. But SCSI has virtually nothing to do with performance, or RAID. No drives outrun their busses, so the bus speed is not a big factor.

Anyway, this thread was about trying RAID with OS X. There's another thread here about What is RAID. And in line with the usefulness that btoneill mentioned, 9 Classic doesn't run from a striped array, probably not from any software RAID set. Just in case you were planning on that. :-)