OS + 3-Disk RAID 5 or 4-Disk RAID 0+1 only?

[tonyalexander]

I've searched the forum extensively but found conflicting opinions on this subject so I'm hoping a new thread can bring some additional points of view. I've ordered a new Mac Pro with the RAID card. Speaking *only* about redundancy (my "backups" will be external and offsite), which setup would you use with four internal drives?

1) One drive for the OS and applications (remember, I'll also have regular external and offsite backups) and the three remaining drives in a RAID 5 array.

PROS:
* Easier to recover from OS issue since that drive is not in an array.
* More storage bang for the buck in the RAID 5 portion.
* Could maybe use SAS drive for OS and SATAs for the array? I've also seen conflicting reports on whether this mix-and-match is possible.

CONS:
* OS volume not redundant (though backed up).
* Slightly trickier to recover from a RAID 5 failure.
* RAID 5 slightly slower overall.

2) All four drives in a RAID 0+1 (or a 1+0 if the Apple RAID card can do it, but I haven't seen definitively that it can) and separate partitions for the OS, media, etc...

PROS:
* OS and application partition redundant. If 1+0 possible can possibly lose two drives before data loss. Otherwise 0+1 protects entire array from one failure.
* Speed benefits all around? OS and applications probably don't benefit as much from the RAID 0 though. Though there may be some gains.
* Quicker rebuild on failure.

CONS:
* Not as efficient use of storage space.
* If OS issue could affect entire array - if array issue may not be able to boot at all from internal disks.

So if it were you - which direction would you go? Is there anything I am missing?

Thanks!

Tony

 

[jconly]

I'll admit, I can't specifically advise you...
But I can tell you what I'm planning on doing, and what I would do, just as some food for thought.

Like you, I'll be having backups of all ARCHIVE data (old imagery). It will exist on two physically different set of drives. One set will be off-line, and off-site. The other will be live and local for nightly backups. Then on a weekly basis, ARCHIVE 1 will backup to ARCHIVE 2.

I am planning on utilizing a RAID (Either 5 or 0+1, don't know yet) for my WORKING data. However, I will not be including the OS drive in this array.
This will be for recent imagery still in the current workflow. This RAID data will be backed up nightly to the archive. My need for RAID is that of speed and protection against drive failure for my mission critical data.

I don't forsee my OS drive requiring the need for speed that my 3GB+ image files do. Yes, the quicker the boot drive, the better for virtual mem use, but I'll pass on the safety risk.

Instead, I will be splitting my OS and my USR data / Apps onto two seperate drives. (Potentially 10K RPM, but probably not due to size requirements) This should speed things up a good bit by giving two seperate data channels. This way the virtual mem can easily be accessed, even when apps themselves are reading and writing to data. I'm assuming that doing so should most likely prevent me from any SATA bottlenecks I very well could see otherwise. In addition, if my OS goes down, I still have ALL my preferences and files intact. All I really need to do is rebuild the OS directory, and things would be relatively smooth sailing from there.

Then, for backing up this data, each drive will be partitioned into two sections. Say 250 OS and 500 USR. Then, each drive will be backup onto it's corresponding partition on the other drive on a weekly basis (perhaps nightly if things pick up around here). However, I am debating on taking a risk using Time Machine to back up my USR data, and going with a smaller, faster RPM drive for the OS. This I'm still undecided upon.



But, back to your question.
If I were to be keeping my OS data and USR data on the same drive, I think I would probably choose option one. (Pending the ability to use both SAS and SATA at the same time) I just feel safer keeping the OS out of the RAID. However, I might actually add another drive to the mix (optical bay) and combine option 1 and 2. Keep the OS out of the raid, and backed up manually to your archive, while maintaining the faster 0+1 capability.


And, not trying to hijack your thread, but I have a question for you.
As I am still trying to decide between 5 and 0+1, would you mind elaborating on how 5 is more difficult to recover from a failure? And just how much slower is it?

 

[tonyalexander]

It sounds like your backup considerations are a bit more substantial than mine. I'm actually willing to lose my array in the RAID 5 setup since all it will be used for is digital copies of physical media (video on tapes, DVD rips to stream to my AppleTV, etc.). I'm fine with having only redundancy and spending a few days re-doing things in the rare event of a multiple drive failure.

RE: RAID 5 vs. 0+1 recovery...

99.9% of the time I think it would just be a speed consideration (and even then with only 3-4 drives not much of one). When you lose a drive in a RAID 5 to recover the system has to do a lot of crunching to grab the parity slices from the other disks and rebuild into a new drive. Most of the time I'm sure this works fine, but it introduces more potential points of failure due to data corruption. When a drive is lost in a 0+1, it's a simple full copy one drive to the new one to recover. Simpler and faster. I could be crazy though. I'm by no means a RAID expert!