It depends a bit on how you set up the drives really. You could make one raidz vdev of 8 drives, losing one of them for parity, or you could make two raidz vdevs of 4 drives each and lose two drives for parity (one for each vdev). You could also do one raidz2 vdev of 8 drives and lose two drives for parity, or two raidz2 vdevs of 4 drives each and lose four drives for parity (2 for each raidz2 vdev). That would give you a bit better redundancy than using 4 mirrors while giving you the same available storage space. The list goes on and on. There are a lot of different configurations you could use with 8 drives, but keep in mind once you add a vdev to your pool, you can't remove it.
Personally, I would not choose to create one vdev of 8 disks, but that's just me. It is important to be aware that when and if you want to replace the 1.5TB disks with something bigger, you need to replace ALL the disks in the vdev to gain the extra space. So, if you wanted to go from 1.5TB to 2TB disks down the road, and you set up one raidz of 8 drives, you need to replace all 8 drives before you gain the additional space. If you do two raidz vdevs of 4 drives each, you need to replace 4 drives to gain additional space. If you use mirrors, you need to replace 2 drives. Or, you can add a new vdev of 2, 4, 8, or however many disks you want if you have the physical space to do so. I believe you can mix and match mirror vdevs and raidz vdevs within a zpool, but I don't think it's recommended to do so. The ZFS best practices guide has a lot of good information in it if you have not read it yet (google). You might have less usable drive space using mirrors, but you will gain a bit of performance, and it's a bit easier to expand your zpool when the time comes. A raidz (1,2,3) can give you more usable space, and can give you better or worse redundancy depending on how you set it up. There is a lot to consider. I hope I didn't cloud things up for you any further or misinform you on something (I'm a newb too, so don't take my word alone on anything). Hell, if you wanted to, you could also do one 8-way mirror that would give you an ignorant amount of redundancy at the cost of 7 drives worth of usable space. It all boils down to personal choice. You have to determine how much usable space, redundancy, performance, and ease of replacing drives mean to you and go from there. ZFS will do pretty much any configuration to suit your needs. eric -- This message posted from opensolaris.org
