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jamesa · May 30, 2004, 09:16 AM

specifically: when your back's against the wall in a RAID 1 mirror - one of the drives have died.

How does the operating system tell you, and how easy is it to rebuild? Did it cover your backside?

Last question - how much of a speed hit do you take for using it?

thx

-- james

jamesa · May 31, 2004, 07:28 AM

so nobody's seen a disk die when they've had the whole thing mirrored?

ok then, what about if you pull out a disk from the raid... the second disk is just not there.

What happens then?

-- james

jamesa · Jun 1, 2004, 09:06 AM

nobody has mirroring set up, or has experience with any of this stuff under OS X?

really?

-- james

wulf · Jun 1, 2004, 09:57 AM

Never used mirroring under software RAID, just striping.

Do you have a specific issue? Are you saying you have lost (or removed) a drive, and the other doesn't show?

jamesa · Jun 2, 2004, 02:45 AM

Originally posted by wulf:
Never used mirroring under software RAID, just striping.

Do you have a specific issue? Are you saying you have lost (or removed) a drive, and the other doesn't show?

no, not that I've lost any drives. I just want to know how OS X deals with losing a drive; whether that occurs from someone just unplugging one of your two firewire drives (ie if you only have one of them, can you just walk up to another computer and plug it in?) or occurs from a hard drive actually dying.

CPU overhead would be cool to know too.

-- james

JMII · Jun 2, 2004, 08:21 AM

Just had this problem come up last week on our xServer... and here is my experience in dealing with the issue:

When the mirrored drive went down the server's performance was uneffected, after all it's just mirroring the drives for backup purposes. The server kept right on serving, both file sharing and web/ftp services were uneffected. Everything kept running off the single drive - no problems or CPU issues that we noticed.

When it happened the warning light on the front of the xServer came on and when you launched Disk Utility is tells you the mirror is out of sync. The problem is that 10.2 "Jaguar" will NOT let you rebuild the RAID while the OS itself is running on the HD, thus you have to launch off the CD. It took a little over an hour to rebuild the RAID on our mirrored 120GB Apple Drive Modules, but each disk only contained about 20GB of used space, the rest was free.

However you had to do the rebuild thru the command line (Terminal) because the Disk Utility application can't seem to handle it properly (refer to Apple Tech doc #106987). It's not difficult, just frustrating... especially since server is off line during this whole procedure. All you have to do is stick in the new drive, type the correct commands, wait about an hour for it to rebuild and then reboot, after that all was fine. We did this at night so our artists and customers would not be effected.

From our research it seems that in 10.3 "Panther" you CAN rebuild the RAID while the OS is running so you'll have zero down time. So if your thinking about setting up a RAID for mission crital data (oh... buzzwords) I'd upgrade to 10.3 beforehand. But it's nice to know that the server is pretty much uneffected by the RAID going down, this way if a drive goes bad over the weekend you don't have to panic... you can just come back Monday and rebuild.

Hope this helps.

- John

Big-C · Jun 2, 2004, 03:19 PM

About 6 months ago I had my boot drive (mirrored) fail and noticed no degredation of service. So smooth that it took me a few days to even notice one of the drives had failed (oops).

As for the rebuild, I don't remember having to run command line arguments to get it working again, but I do remember that the re-sync took a fair amount of time.

All in all, I was very happy with the fact that the machine kept running without a hitch. I've heard good things about SoftRaid, but I've never used it. We're getting 3 new xraids within the next few weeks - I may try that out. I've heard SoftRaid will let you resync while the machine is up... Maybe someone with more experience with SoftRaid can comment on that though.

Anyway, Apple's software raid worked perfectly for me.

-Chris

Millennium · Jun 2, 2004, 04:00 PM

If one drive fails under RAID 1, there should be no degradation in performance. That's what RAID 1 is designed for: if one drive fails, the other takes over. You pay dearly for this, because essentially you only get to use the space on one of the drives involved. But when nothing less than complete fault-tolerance will do, it's the way to go. You should, however, have some kind of monitoring utility to let you know that one of the drives has failed, so that you can replace it before the other one goes down. Otherwise, you might not know that any of the drives is dead until they're all gone.

RAID 0 (striping) is another matter entirely. This one trades stability for speed. You get virtually twice the data throughput if you're using two drives (triple for three, quadruple for four, etc.) but if any of the drives goes down, the others become useless. If you have RAID 0, then backups become even more important, because there is a greater risk of needing to use them. However, as long as you keep good backups, then there's not much to worry about.

There are various other RAID schemes which try to mix these two schemes. None of them get the complete fault-tolerance of RAID 1 or the huge performance gains from RAID 0, but they can give you a significant boost in both areas at once. However, Apple does not support any of these in software right now (though they might on the XServe RAID), and even if it did, they usually require more than two drives.

NOTE: Don't try to RAID two partitions of the same drive together. It's theoretically doable, of course, but RAID 1 in this situation is pointless (since they're on the same drive, both will be knocked out if the drive fails) and RAID 0 won't work (since the partitions share the same disk, only one can be read at a time, and the ability to read multiple drives at a time is what makes RAID 0 work).

Art Vandelay · Jun 2, 2004, 04:05 PM

Originally posted by Big-C:

All in all, I was very happy with the fact that the machine kept running without a hitch. I've heard good things about SoftRaid, but I've never used it. We're getting 3 new xraids within the next few weeks - I may try that out. I've heard SoftRaid will let you resync while the machine is up... Maybe someone with more experience with SoftRaid can comment on that though.

You don't need a software RAID with an Xraid. The Xraid is a hardware RAID. It's all done within the hardware controllers, which makes it much faster and better than any software RAID.