Feb 132010
 

For some reason, I want my future file server to have space for 8 hot-swap HDD. Those HDD will house the RAIDZ cluster of my ZFS array (although that particular terminology sounds odd). Why do I want hot-swap? Because I can. It costs about $100 for a drive cage for 4 SATA drives. That’s +$25 per HDD, but it’s nice. That’s all. It’s Nice To Have.

I said RAIDZ, but I might go with raidz2, or perhaps raidz3. They can withstand 2 or 3 simultaneous disk failures. The different levels require a different minimum number of disks. ZFS clusters are recommended to contain between 2 and 9 HDD. If you need more than 9 HDD, create a new cluster.

  • raidz – 3 HDD minimum
  • raidz2 – 5 HDD miniumum
  • raidz3 – 8 HDD minimum

Taken from ZFS Best Practices Guide.

The following table shows how much disk space I would have in each arrangement of parity, disk size,and disk quantity.

  Disk size (TB)
  1 1.5 2
Num Disks Parity Space Available
3 1 2 3 4
4 1 3 4.5 6
5 1 4 6 8
5 2 3 4.5 6
6 1 5 7.5 10
6 2 4 6 8
7 1 6 9 12
8 1 7 10.5 14
8 2 6 9 12
8 3 5 7.5 10

The plan is to start with four HDD, so at most, I can get raidz. Which would give me 6TB of space with 2TB HDD.

I’ve also been thinking about how many SATA ports should my motherboard have. Six is pretty common. But I also plan to have a DVD drive. I also plan to boot off a non-ZFS array. That will require two SATA ports, because I will use gmirror on that.

This means I need 3 SATA ports for the system, plus one per each HDD in the array, for a total of 11 ports, if I go with an 8-HDD ZFS array.

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