unique RAID configuration situation
Here's a challenge for all the raid-heads out there. I have a MDD G4, which as you may know has one ATA100 channel and one ATA66 channel. I am going to do either 0+1 or 1+0 with 4 drives. I'm pretty sure that in and of themselves, 0+1 and 1+0 have the same redundancy when using only 4 drives.
Should I do 0+1 or 1+0? How should the drives be spread accross the controllers? (Both on one controller, or one of each on each controller)?
First I thought this would be simple enough to figure out but the more I thought about it the more the different factors seemed to balance one another out. For example, first I thought that putiing a RAID 0 array on one controller would be silly... but really, the software has to wait on both controllers anyway when doing the raid 1.... (assuming 0+1).... wait a second... arrr my brain hurts
Should I do 0+1 or 1+0? How should the drives be spread accross the controllers? (Both on one controller, or one of each on each controller)?
First I thought this would be simple enough to figure out but the more I thought about it the more the different factors seemed to balance one another out. For example, first I thought that putiing a RAID 0 array on one controller would be silly... but really, the software has to wait on both controllers anyway when doing the raid 1.... (assuming 0+1).... wait a second... arrr my brain hurts
Comments
Cheers,
Martin.
Then you are going to also make sure that you have hamstrung your ATA100 even further by tying it to the performance of a ATA66... making sure that you get less performance than your worst component...
Executive summary: if you really need RAID then get hardware RAID. If you can't afford hardware RAID, then you probably don't need it, and you should be thinking more of short-period backup (think rsync).
Originally posted by Karl Kuehn
RAIDing more than one drive on an IDE channel is not a good idea, your performance will absolutely suck. Just having a second drive on a channel reduces performance, and trying to use both all the time kills the performance.
In your situation you can have one or the other, performance - striping, or reliability - mirroring. Choose which is most important.
You would then apply that choice across the two controllers. I would run them at the same speed ( ATA 66 ) which is just fast enough for accessing one disc at a time.
If you need the extra space you can run JBOD on each controller. That just pretends that the two discs on the channel are one disc ( giving you lots of space ). This is risky ( loose the primary disc and you loose both ) but the mirroring will save you ( unless it is a software corruption ).
Im not sure that the ATA 66 controller is suitable anyway. It is designed for relatively slow devices, cdroms, which only get up to 7.5 mb/s. It probably doesnt have the resources you want to do sustained high speed transfers.
You should at least invest in an ATA controller which gives you two or four matched channels, that are intended for high speed usage.
Thus, if what you are looking at isn't in the cache, and the computer has to call it up from the drive, there is little or no difference between ATA/66 and ATA/100 drives.
Sticking an ATA/100 drive on a ATA/66 bus is neither here nor there because the majority of PATA drives out there are ATA/133 no anyway, so even on the 'faster' 100MBps bus you're still not going to get the full potential of the 133MBps drive.
I'd concentrate more on the spindle speed and the amount of cache on each drive. You'll want a minimum of a 7,200 rpm drive and a minimum of 8MB of cache memory on each drive.
I believe that when you connect two drives to a single channel on a MDD the two drives contend the total bandwidth ? so what you'll end up with is two ATA/50 drives (if such drives existed). This effectively negates the whole point of RAID level 0, so you only really have one choice ? span each stripe set across the two available channels, and then hope that mirroring doesn't cause cause too much contention (which lets face it it's going to).
The G5 is the first desktop computer that Apple has produced that is able to properly support RAID directly out of the box ? it has two completely independent SATA channels, which means that a 300MBps stripe set is (in theory) achievable.
So ends the theory. As someone who has tested both means of setting up RAID O on a MDD years ago, I can concur that hardware RAID is the only way to go. Software RAID on a MDD is a complete a waste of time (with regards to performance gains).
In addition, remember that RAID 1 only protects you from failed drives. If a file becomes corrupted or is accidentally deleted, the computer will immediately duplicate this problem onto the mirror.
My advice would be forget the RAID 1 option and buy a hardware RAID controller ? then back up all your data regularly onto DVDs. RAID 1 really doesn't have any place on the desktop, and it can be costly to acheive the benefits of RAID 0. With a dedicated RAID controller and two suitable drives you might just be able to achieve 266MBps (cache burst).
Unless of course (like me) you simply enjoy pottering around with your computer ? in which case try all the methods and see what works best for you. IMO, the MDD series is the finest "enthusiasts" computer that Apple has ever built. Four hard disks and two optical drives ? that machine packs one hell of a punch.
Good luck.
The question I pose is: Why would Apple use two controllers of different speeds ( 66 and 100 ) when they could just use two 100s, like every other manufacturer?
They obviously have the design, so it must be something else.
My suspicion, which would require testing, is that the 66 controller is only intended for use with optical drives. As a result they were able to tune it down, and cut costs. My gut feeling is that that particular controller wont perform well when connected to high speed devices ( hard drives ). It is only designed for use with devices which can do 7.5 mbps sustained, not the 40 mbps that hard drives of the era could manage.
As for putting two drives on a single channel. ATA is pretty suck when it comes to managing requests on the channel. Only one drive can be accessed at a time, and each request must be carried all the way through ( contrast with SCSI which can issue requests to multiple drives and then pick up with them later ). Because of this ATA doesnt perform very well for highly concurrent disk access ( operating systems work hard to hide this ), which is pretty much what you are going to be doing with RAID.