The Logic Off Topic list

On 17.09.2006, at 18:21, garygenn wrote:
> I saw a product that lets you mount up
> to three more hd in your mac and it was talking about setting up
> as a raid conf. My question is would a raid set up help
> perfromance   and what raid conf is best. I saw something about
> raid 1 being pretty useful. I would like to have one hard drive
> mirror my system hd, then the other two set up to work together. I
> dont know much about it

Crash course:

No RAID
One or more drives, all of them independent. That is what you have  
now. Easy to manage, fast, no security. If one single drive gets  
involved for several operations like supporting the operating system  
and recording audio it becomes slower. If the drive fails the data  
might be lost.

RAID 0
Data gets written to and read from all drives simultaneously. It is  
the fastest type of RAID and has no redundancy which means no  
security. If one drive fails all data of the whole RAID is lost. Good  
for streaming data like video but in my opinion not necessary for  
recording or playing back audio.

RAID 1
That is mirroring. Two drives are involved which bear almost the same  
data. Data gets written to and read from both drives but not  
necessarily simultaneously. Reading can be faster than with one drive  
if the system is clever enough to use the data from that disk which  
can deliver them faster. Writing might be slower than with one single  
disk. You get a lot of security because if one drive fails you should  
be able to work with the other one alone until you replaced the  
broken drive. I think the reduced writing speed does not really  
affect recording capability. Mirroring is certainly an option for  
people who do not always have a solid backup system or want a short  
interruption time in case of a hardware failure.

RAID 3
It stripes the data in the same way that RAID 0 does but additionally  
writes parity data to a dedicated parity drive. In case one disk  
failes the controller can restore the data based on the parity  
information to a replacement drive. RAID 3 is designed to employ  
every disk on every input/output operation. Unlike RAID 0, RAID 3 is  
a redundant system which can withstand the failure of one drive and  
has little diminution in performance if one disk fails.

RAID 5
That one takes a different approach towards striping and parity  
storage to better handle applications that barrage the system with  
many, small input/output operations. Unless RAID 0 and 3 it is  
designed o engage all drives  in the array at the same time on  
different reads and writes. In/Out per second is higher than I/O of  
RAID 0 and 3. Performance is remarkable slower if one disk fails.

Now, what does all that mean to you? Relax and see it practical ...

No RAID at all but a good backup system is good. You can have one  
system disk which bears also your applications, One disk for sample  
libraries and ond disk for audio (and recording of course). Flexible  
and cheap and a modern computer can handle that.

If you want more security, the question is - what for? For the system  
or for your audio data? They should never be on the same disk so you  
have to mirror one or both of them.

If you want speed, speed, speed, you might go for RAID 0. But I don't  
see an advantage for audio. Although the description of RAID 3 sounds  
promising I have never seen a RAID 3 outperforming a RAID 0. That  
might have been been the kind of data but however, RAID 3 is rarely  
used and people who depend on data aren't that stupid.

If you want a fast and reliable redundant system for your audio data  
and if mirroring is not enough for you you should go for RAID 5 which  
is a widely used standard. You need at least three disks for that.  
And because of the parity you lose 25% of the capacity or one drive,  
whichever occures first. In other words: with a minimal 3-disk system  
you get only the capacity of 2 disks. The relation becomes better  
with each drive you add.

Important:
There are software RAID's and hardware controllers. If you want to be  
really secure do not rely on a software RAID although some of them  
are quite good (the SUN solution for example). I do not know Apples  
built-in software RAID. However, a software RAID draws some  
performance from your computer because it cannot work independently.  
But it is cheap.

I do not know much about RAID 7 which Otto mentioned in his mail but  
be aware that most "RAID people", in this case the people who want to  
sell you a system, speak about SCSI. Our Macs don't have that built  
since a while. SCSI drives are fast, small and expensive.

There are other RAID levels for bigger systems (mirrored RAID 0 for  
example) but I think they are not in out focus here.


Hope that helps,
Peter Ostry