0 Replies Latest reply: Aug 23, 2013 6:27 PM by user9027599 RSS

    ZFS Usable Capacity Question

    user9027599

      The question is about usable space in a ZFS 7320.  System has two shelves - total (44) 900GB and (4) write-accelerators, so around 40TB raw.  The summary per ZFS software is as follows:


      (1) Pool

      Striped Log

      Double Parity

      __________________

      Usable Data - 28.3TB

      Parity - 5.62TB

      Reserved - 480GB

      Spare - 1.64TB

      _________________

      Data & Parity - 42 Disks

      Spare - 2 Disks

      Log - 4 Disks

      Cache - 4 Disks


      Snapshot can be seen here - http://systemsmarketing.com/ZFS_Snapshot.png


      After spares, parity and reserved there is 28.3TB available.  User wants to add another shelf but wants to be sure it will give him what he needs. 

       

       

      Adding a shelf with (24) 900GB is 21.6TB raw.  The goal with that shelf is to be well over 40TB usable, hopefully at least 45TB.  What we are trying to determine is, after adding the shelf, wiping everything and restarting with one pool, how much usable will be added?  We assume it will still be two spares so the 1.64TB won’t change.  The big question is the parity and whether that allocation is linear.  We're assuming not as there is probably some initial overhead in that figure, but don’t know.

       

      Our experience with Sun formatting on earlier drives is that firmware can account for as much as 15% but don’t know if that applies to these ZFS HDs.  If so that would take off 6TB of the initial 40 which could explain most of the differential (expected had been around 35TB).  If that 15% came off the  21.6TB shelf we would be starting at 18TB and any significant parity loss could be problematic.


      Input on this question would be appreciated.  Thank you.