RAID rant

[Bob_Campbell]

😡 I find it hard to believe that I’m the only one out there who insists on mirroring or otherwise ensuring that my system disks are protected. Yet, I don’t seem to see anyone on OS X sites mentioning the following:

Despite Apple’s claim that their Xserve’s come with “Up to a half a terabyte of disk!” the space cannot be reliably used that way.

The only thing I can applaud the RAID setup on OS X Server about is that it lets you create them before installing. Presumably (I haven’t been able to test this) if the system experiences a failure, I can boot from a cdrom, or part of a mirror and recover.

I take issue with the following, however:

1) The RAID setup only allows full-disk mirroring or striping.
This has a few problems. Namely, I cannot partition the disk in any way if I choose to mirror or stripe the disks. Thus, I cannot conserve space by limiting the system partition or separating a data partition so as to mirror my system area but stripe my data area. Nor can I separate my swap area if I mirror or stripe the disks. For example, I had hoped to create 8 GB partitions across the bottom of all four drives, and mirror them; one mirror for the system, one mirror for the swap area (a waste, but it ensures integrity at least) then striping the top of each of the four as a data area that does not require integrity of the filesystem. Not being able to partition the disks first is really quite a drawback (and I can’t believe wouldn’t be difficult to fix!)

2) Striping is dangerous, Mirroring is wasteful, nothing else exists.
Striping all four drives into one huge partition (460 GB) is dangerous, especially in the case of IDE drives, which have a lower MTBF than scsi drives. Mirroring two 120 GB drives just for the system is wasteful if the system can live happily in 8 GB. If I settle for mirroring, I get 230 GB in two 115 Gb filesystems out of a possible 460. If there were at least RAID 5 (I know, software-based RAID 5 sucks, but with two 1 Ghz processors in the thing, I’ll take it!) then I’d only be losing 115 GB – 25% instead of 50% and I could live with the single filesystem (despite the speed gain of a separate swap area).

3) No RAID 1+0 or 0+1
If all they’re going to offer is RAID 1 or 0, the least they could do is allow us to stripe a mirror set or mirror a stripe set. At least in this mode, I could make one 230 GB filesystem instead of two 115s.

They seem to have the hardware idea down well; four independant IDE controllers, one for each disk so it can get the most throughput, but they really need to come through with more options for utilizing it!

Bob Campbell

[dave@mmu]

we have all 4 drives at 120Gb, 3 striped into 1 and 1 drive for the system..
Seems to work ok..

[aaronfreimark]

I hope you make really good backups. I don’t trust Apple’s RAID at all. It doesn’t alert you when a mirrored drive has failed. Rebuilding a mirrored drive must be done off-line, making “hot-swap” useless. A client’s setup was somehow magically switched from mirrored to striped, making it completely unreadable. It is impossible to rebuild on an XServe running Jaguar Server.

I read somewhere that RAID was boolean — it works perfectly, or not at all. I only recommend external RAID systems for my clients.

[stmoddell]

So I’m not lucky enough to have an XServe yet. But in all my current servers, I only use hardware RAID. Never had any luck with software RAID, so if the Apple RAID is software based, I wouldn’t trust it.

I’m surprised that partitioning the arrays isn’t avaialble. Sound like this is a product in the early stages of life.

[bcirvin]

anyone working with apple server hardware/software in a multi-platform server environment will see good and bad points.

i find that it helps to keep in mind that currently, apple’s stuff is definitely on a steep development curve, and probably not ready for core enterprise deployment yet.

HOWEVER, for a lot of organizations (like many of my clients), apple’s attractive price/performance ratio is a big selling point.

about software raid – as of 10.3.3, one can now rebuild software raid arrays on the fly (ie, in a hot system), apple says. also, you CAN be alerted to hardware issues by making use of the Server Monitor app – i’ve used it to spot a degrading software striped raid on an xserve, and fix the problem before it got messy. btw, this app only works with xserves, not towers or other apple units.

but, please, steve, give us an xserve with hardware-controlled raid!

blake/

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