Nexenta scale and cluster

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Hans de Leenheer blogged about Nexenta again Smile good post about Nexenta failover techniques.

1) Disk management:
The first part of failover management is still plane old ZFS. It’s the way disks work together, how I/O is handled and how data corruption is avoided.

  • Storage Pool: a set of (equally configured) vDEVs
  • vDEV: a set of disks with a certain protection level
  • RAIDZ: first form of physical disks protection level. Examples:
  • RAIDZ-5: 4 disks data, 1 disk parity
  • RAIDZ2-6: 4 disks data, 2 disks parity
  • RAIDZ3-9: 6 disks data, 3 disks parity
  • Mirrored Disks: second form of physical disks protection level. Here too you have multiple choices of failover (dual-mirror, triple-mirror, …)

Stripes: writes are striped across all members of a vDEV, therefore the speed/bandwidth will be truncated by the slowest member. To add bandwith/IOPS, add more vDEVs. But this is for parallel IO streams! One IO stream = 1 vDEV.

RAID5 write-hole: ZFS is protected against the RAID5 write-hole because it does copy-on-write instead of read-modify-write. If in case of a power failure some blocks are not completely acknowledged (incl parity-check), nothing would have happened.

Physical disk-enclosure failover: if you design smart, you could even design for disk-enclosure failover by aligning the vDEV physical disks vertically (1 per enclosure)
2) System Scale:
The next step is in-system scale. It’s not really high availability but an equally important step in the story going forward. Because you benefit a lot by a bigger bandwith with ZFS you’d want to "scale-out" first inside your system and then go to "scale-up". Here you find a simplified example of that.

So instead of daisy chaining the 3 first nodes and then adding a second and 3rd loop I did it the other way around so you’d have 3 loops of 6Gbps SAS connectivity from the beginning.
Remark: this is a theoretical model. I would have to doublecheck with the solutions team before designing this for a customer.

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1 thought on “Nexenta scale and cluster”

  1. In your pictures, why do you have one controller connected to the second controller in the same chassis?

    what are the benefits? Surely they can communicate over standard backplane?

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