Mac OS X Server 10.6 - Apple's software mirrored RAID worth it?

I am installing an Intel Xserve (Quad core Xeon) with Snow Leopard Server (10.6) on two 80Gb 7200rpm SATA HDs.

I created a mirrored RAID set using Disk Utility with those two drives, all went fine.

I was then asking myself if this is really a good idea. I know that an hardware RAID system would be better, but what about this software RAID?

Have you any feedback on this? Will it work fine if one HD breaks down? Does this affect performance?

[UPDATE]

In short: Hardware RAID is better than software RAID which is better than none.

Thank you all for the answers, they were very helpful.

Especially Gordon's script to monitor failures. As Apple's software RAID is pretty silent about a drive failure.

Solution 1:

My personal preference is always for hardware RAID but would use software over none. I am also aware there are some who have the opposing view. Yes, it will continue to function if one of the drives fail. That is the main reason to use RAID. The "R" stands for redundant.

Software RAID must impact performance because the CPU is having to do more. However, in reality if that difference becomes noticeable you're pushing the server far too hard, which will result in more problems than the loss of a few CPU cycles. e.g. Heat will become a serious concern.

One question I would ask is why such tiny drives? For very little more you can get much larger drives. If nothing else, 80GB drives are becoming quite hard to source, which may be an issue from the future maintenance point of view.

Solution 2:

I'll second SvenW's warning about silent failures; if anything, it's a little too good at surviving a drive failure. I've seen the aftermath of a couple of servers that had one drive drop out of a software mirror for some reason (I suspect not coming ready in time after a reboot); everything works fine off the remaining drive until, several months later, something goes wrong with THAT drive -- and it switches back to the drive that glitched the first time, and the last few months have vanished.

Here's a short shell script I whipped up to fix this. Substitute in your email address, save it as something like /etc/periodic/daily/150.check-raid, make it executable, and it should mail you a warning (at 3:15 the next morning) if the raid ever degrades. To test it (strongly recommended in case of spam blocks, etc), plug in a couple of disposable drives (USB keychain drives, whatever), mirror them, unplug one, leave the other overnight and see if you have a warning in your mailbox in the morning.

#!/bin/sh

# This script checks for any degraded/offline/failed/whatever software
# RAIDs, and if any are found emails a note to an admin.  To use it,
# replace the ADMIN_EMAIL value with your own email address, drop it in
# /etc/periodic/daily, and change the owner to root.  This'll make it
# run its check every morning at 3:15am.
#
# Warning: this script doesn't check anything other than software RAIDs
# built with the Apple (i.e. Disk Utility) RAID tools.  It does not check
# any hardware RAIDs (including Apple's RAID card), or even any third-party
# software RAIDs.  If "diskutil listraid" doesn't list it, it's not going
# to be checked.
#

ADMIN_EMAIL="[email protected]"

if diskutil listraid | grep "^Status:" | grep -qv "Online$"; then
    diskutil listraid | mail -s 'RAID problem detected' "$ADMIN_EMAIL"
fi

Solution 3:

I have good experience with the software raid, but I only use it as the system drive. Be sure to use Server Monitor or Disk Utility though to check for the drive status, as at least in MacOS 10.4 the system is quite silent about a failed drive. I am not sure if this got better in 10.5/10.6, this is something on my test list. One thing I really miss though is RAID5, when you have more than two drives all you can do is striping.

In my case, performance was not affected noticably, but again, it's used only as a system drive, with an FC RAID for data.

When you are still in the testing process, make sure you test failure/recovery by removing a drive so you know how it's done in case of a drive failure.

Solution 4:

The XServe RAID card is $699 - although I much prefer hardware RAID I'd say that OSX's software RAID is good enough to seriously undermine the justification of such an expensive hardware controller.

Basically don't worry about it, spend a fraction of this money on a =>80GB USB disk and leave it to Time Machine to give you a recoverable backup if you lose your mirror.

Solution 5:

Modified version of Gordon's script to check Apple's hardware RAID card.

#!/bin/sh

# This script checks for any degraded/offline/failed/whatever software
# RAIDs, and if any are found emails a note to an admin.  To use it,
# replace the ADMIN_EMAIL value with your own email address, drop it in
# /etc/periodic/daily, and change the owner to root.  This'll make it
# run its check every morning at 3:15am.


ADMIN_EMAIL="[email protected]"

if raidutil list status | grep "^General" | grep -qv "Good$"; then
     raidutil list status | mail -s 'RAID problem detected' "$ADMIN_EMAIL"
fi