View Full Version : RAID 5?

07-28-2001, 10:19 AM
I don't know if this is how it works, but my friends at work (which all have PC's at home) were talking the other day about their RAID set-ups. I think they called it "RAID 5;" where you have like 3-100GB drives, but can only use 200GB of space to store things on because they are all working together to store data - so that if 1 of the drives go bad, they rebuild themselves? Is that kind of backup system possible on Macs with some sort of card for EIDE drives?


07-28-2001, 09:41 PM
When they talk of RAID-5 they are not talking about EIDE/ATA33. For any RAID you really want SCSI. I think there are some hardware IDE/ATA 'RAID' cards but I am not sure how well they work. Since Mac PCI IDE/ATA cards mimic SCSI it is possible to use SOFTRAID with these. I think it can work for stripping (RAID-0) to get a bigger drive (using two 40GB or two 100GB drives to equal 80GB or 200GB). I've heard that mirroring (RAID-1) does not work so good with this setup. This goes for Mac or Windows.

Mirroring is like RAID 5 in that it offers greater redundancy, but you pay a bigger pentality.
1) You loose 50% of your space
2) You do not gain any speedup in write throughput
3) You only gain a little speedup in read throughput

Of course DATA can be EVERYTHING so most servers or systems with mission critical data run this way (or in RAID-5). I have never done this as I normally run two or more systems. They will often have RAID-0 and I will make sure they all have the same data more or less. If any one system crashes or has issues my data is safe.

RAID-5 is normally done via specialized SCSI cards. I've never see a good option for the Mac in this area, but system X may change all that. Normally RAID-5 and SCA drives are ONLY for servers. The average joes does not have a clue about this and does not need it anyway. RAID 5 needs a minimum of 3 drives and combines the features of RAID-0 (speed & size) with the features of RAID-0 (reduncancy). It is possible that SoftRAID or other software solutions could work with a RAID 0+1 or even RAID 5, but I havent seen this. It is probably not reliable enough to work in software and reliablity would be the MAIN reason for going this route http://macgurus.com/infopop/emoticons/icon_smile.gif Also any REAL server system runs backup tapes at least once a week and most of these tapes are carried off site in the event of a disaster. So RAID-5 is really only the first tiny step in data integrity.

Life in the fast lane leads to:
The Resteraunt At The End Of The Universe

07-29-2001, 05:16 PM
The concept of a RAID (Redundant Array of Independent Disks) does not, in itself, imply any particular communication protocol. Hardware SCSI RAID boxes and IDE/ATA RAID boxes exist to implement all higher levels of RAID (3,4,5). The proprietor's of this establishment have tested Hardware SCSI RAID boxes and not found any yet that do not have firmware conflicts with the Mac Operating System. The main reason that RAID subsystems almost imply the use of the SCSI communications protocols is that SCSI is much more flexible and scalable than IDE/ATA. A single SCSI controller offloads almost all I/O function from the main CPU and can communictate synchronously with up to 14 other devices. An IDE controller requires main CPU time, can handle only two devices and communicates with only one of those at a time. You can somewhat mimic the functional scalability of SCSI by complexing multiple IDE controllers together with other integrated chips on an expensive and complex hardware backplane.

Software RAID options available:

RAID 0 (data striping across n drives)--read and write speed theoretically n times faster
RAID 1 (data mirroring across 2 drives)--reads theoretically 2 times faster, writes same speed as single drive
RAID 0 +1 (mirroring a striped volume to another drive or striped volume)--this is said to be in the future for Intech's SoftRAID

IDE/ATA RAID 0 and RAID 1 (striping or mirroring across 2 drives)--uses two separate controllers on the same card to to be able to communicate with one drive one each of the separate ATA busses at the same time (sort of like an ATTO dual channel SCSI card except that the ATTO could handle 24 devices and scale speed accordingly up to the saturation point of the SCSI bus or the PCI bus)

RAID 5 is done with a dedicated hardware backplane and is very different (data striping and Parity data striping across n (at least 3) drives, n + 1 drives required for one hot-swap spare)--data and Parity data are striped across n drives; if one drive fails, read/write data is rebuilt on-the-fly from the other data and parity segments and the failed drive is rebuilt onto the spare. Max. theoretical speed increase is only n/2 since only n/2 drives are accessed at any one time. Command overhead and Parity calculations also decrease throughput significantly.