The Logic Off Topic list

On 22.09.2006, at 05:08, garygenn wrote:

> Hey Peter I was looking at some help files on my Mac and ran across  
> this after reading
> your raid post again . thought you might be interested:
> After you create a RAID set, the RAID software manages storing  
> files on the disks in the set
> for you.
>
> If you have sufficient disks, you can create more than one RAID  
> set. For example, you can
> set up one set as a mirrored RAID set to provide data redundancy,  
> and another set as a
> striped RAID set to provide faster performance.

Ah, good to know. I had never a software RAID on the Mac. Good  
feature, certainly. Although I don't think that people who need two  
arrays would do it with a software RAID. And I wonder how the drives  
are connected ...

However, their suggestion is close to an ideal stationary setup for  
speed and security which would be:

1. A RAID 1 (two mirrored disks) for operating system and applications.

2. A RAID 0 (two or more, striped) for the data.

3. A second RAID 0 which is a mirror of the other one (remember, RAID  
0 has no redundancy at all).

That setup is known as RAID 10 or RAID 0/1 because type 0 and 1 are  
involved.  It requires at least 6 disks. Data gets written to all  
almost simultaneously. Expect some noise.

I think we can state that everybody who wants speed and security for  
video, audio or fast databases wants such a system but few have it.


> I guess we found out two things about my mac  1) you can have two  
> different raid set-ups
> and 2) I guess it is a software controller on here

Yep. You obviously already know what to look for.

If you are not a software technician and do not have a technician  
available all the time you may be better off with hardware. If a  
drive fails in a hardware array then a light at the drive bay goes  
on, the drive gets disabled, you change the disk and compose a short  
song or do some smalltalk while the RAID recovers. Not that easy in  
software. I don't know the Mac solution but I remember well the few  
occasions where a disk in a Unix software array failed (Internet  
server stuff), the sweat and swearing of the admin and the great  
relief after he got it fixed.

Of course a hardware controller can fail too. In this case you are  
lucky if a replacement controller sees the array as it were its own.  
Few people discuss that with the manufacturer before their purchase.  
Some people even don't trust the manufacturer and buy two systems.  
But that starts to go into dimensions beyound our usual budget.

---

I guess we are almost through, let me add some final notes:

There should always be at least one spare disk available. Some  
systems need identical drives, in that case it is wise to get more  
spares in advance. Running such a system is not cheap ;-)

If you setup a RAID 5 or 1, take some time for a test: initialize the  
array, write some data to it. Then turn everything off and disconnect  
one disk. Start the array and computer again and see what happens.  
You ought to know the situation and the recovery process for the time  
when it happens in reality. A manufacturer of a disk maintainance and  
recovery program had the following statement on the first manual  
page: "It is not the question if a drive fails. The question is when."

Second, be aware that a RAID is no substitution for a backup. If the  
controller fails you can be out of business. Some big environments  
consist of identical hardware RAIDs on different locations without  
other backup, certainly not what we are talking about. You need a  
reliable backup software with easy and fast recovery of single files,  
folders and probably whole disks. Best for that is most likely a  
sequential backup via Ethernet with Retrospect (server version),  
today rather to harddisks than to tape. You can run Retrospect on an  
older computer, I would say the speed is not very important in this  
case. Of course the machine should not be too slow because you might  
want to backup other machines too. Retrospect Server is rather  
expensive, a Windows version is available.

The latter closes the circle. Since backup is required I would rather  
go for a good backup solution first which might need more than one  
disk but not necessarily a RAID although (software) RAID 5 would be  
fine. A RAID for production might be necessary for video and fine for  
audio but has definitive a lower priority.

Running a RAID is more fun than running a backup system but well, it  
is, hm ... your choice, your data ;-)

___
Peter Ostry