We've bought some Xserve and Xraid hardware for our new production and playout system. The Xraid came pre-configured with 2 Raid 5 partitions approximately 2.7 TB in size using 500 GB drives. Anyways, I'm now setting up Xsan to run on these machines so we can add more Xraids in the future. The question is how should I configure it?

The San needs to store its journalling and metadata, it needs to store it in the San itself. The manual says that it can be stored along with the user data, but it then says that volumes containing meta-data cannot be grown later on. I could set up a separate Lun for this but the minimum size would be 500GB without Raid or a whole TB with. A complete waste when the manual says it only needs 10-20GB. Also when I choose to upgrade, I'll lose any extra storage in the new Xraid as all the Luns have to be the same size.

What's the best way to do this, the administrators guide isn't particularly helpful.

Erm, are you trying to setup a Clustered Filesystem?

We needed something that would allow us to grow a single volume over time by adding Xraids.

I did it at home with LVM under linux, but it seemed Xsan was the answer for Apple.

Ahh..well .. you could install linux on the Xserve machines, or attach linux machines to the Xraid. The fact that OSX doesn't have anything like LVM built in.. or the fact that Xsan takes up that much disk space for journaling is kinda lame. I wonder where the code for Xsan came from.

Well, a SAN can't really use host-based LVM because the data may, and probably will, be accessed from multiple hosts. If it isn't, or at least isn't designed to be, there's not much point in having the SAN to begin with.

If I read the admin guide correctly, it seems to me that you could reconfigure the LUNs on the Xraids to be much smaller and then aggregate them into larger volumes.

Or, rather (after reading the Xserve guide), slice the current arrays into smaller LUNs.

That's why Sistina^H^H^H^H^H^H^HRedHat invited CLVM

Apparently they don't recommend slicing, something about a performance hit and increased wear on the discs. I've decided to leave it as the following...

Originally 14 x 500GB discs, 7 per controller

The new config:

2 x 500GB drives as a RAID 1 pair on controller 1 for metadata
6 x 500GB drives as a RAID 5 set on controller 2 as the user data volume

The RAID 5 set is 2.27TB which means I'm only wasting 270GB as the size limit on the LUN is 2TB

This leaves 1 disc from controller 2 and 5 discs from controller 1. I'm therefore leaving it at 2TB until the new financial year when I'll buy a 3.5TB XRaid (7 x 500GB drives on controller and nothing on the other) whip a disc out of it and put the 6 other discs on the other controller. That would result in 2 x 6 disc RAID 5 arrays which I can add to the SAN.

I should therefore have 6TB at my disposal plus a cold spare drive module.

It's not quite what I wanted, but it's probably the cheapest solution which will give me a working system now.