All recent Macs support software RAID in the BIOS, including the Xserve. However, it can be difficult to set up, and there is very little advice on best practices for how best to use it.
The key issue is that unlike many other operating systems that support software-based RAID, you can only RAID entire drives, not individual partitions.
Thus on the Xserve G5, there appear to be two choices:
- Build a mirrored RAID 1 out of the first two drives, install the OS on it, and use the third drive as a backup drive that is easily removable, or as a hot spare. This solution appears to offer the most reliable boot, as if the first drive fails, the second drive is still bootable. However, the system files and data share the same partition, making backup and restore more difficult.
- Install the OS on a non-RAID drive, and use the second and third drives for data using mirrored RAID 1. The advantage of this is that you can partition the first drive into several smaller volumes, making it easier to restore the system. The disadvantage is that if the boot drive fails, you can't boot until you recreate it. Another disadvantage is that the Mac OS X Server 10.4 installer doesn't easily allow you to separate system files from data.
I've tried both approaches, but I'm now leaning toward second. My first drive has 4 partitions -- 3 identical 30GB partitions, the remainder a spare volume. The first 30GB partition I boot from and is my working OS. The second partition is a clone of the first, in which I use Carbon Copy Cloner on a regular basis to keep up with the boot drive. The third is for experimenting, but for now I keep a copy of the OS X Server in stand-alone mode before I made any changes or added any services.
One particular advantage of this scheme is that I can use Carbon Copy Cloner to update the clone on the second partition, and then use Disk Utility to image it without rebooting the system (you can't image a boot volume with Disk Utility). This image I then store on an old iPod. This means that if anything goes wrong, I can quickly restore the disk image into a replacement boot drive without touching the RAIDed data drive.
There is also an option in Carbon Copy Cloner to support the creation of something called ASR (Apple Software Restore) disk image which can be burned onto a DVD-RW which sounds like it would make reinstalling even easier.
The challenge of using this scheme is separating out the data from the boot volume, which I will leave for future blog posts.
Comments