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How FreeNAS partitions ZFS disks

 

By default FreeNAS doesn't use whole devices for ZFS vdevs, rather it adds a guid partition table (GPT) with 2G swap slice and the rest for ZFS. The 2G swap size can be defined on the TBD page.

# gpart show da5
=>        34  5860533101  da5  GPT  (2.7T)
          34          94       - free -  (47k)
         128     4194304    1  freebsd-swap  (2.0G)
     4194432  5856338696    2  freebsd-zfs  (2.7T)
  5860533128           7       - free -  (3.5k)

If the size of the swap is reduced to zero, then the vdev still created with a GPT but no swap slice. This setting has a strong warning of:

Swap size on each drive in GiB, affects new disks only. Setting this to 0 disables swap creation completely (STRONGLY DISCOURAGED).

# gpart show da5
=>        34  5860533101  da5  GPT  (2.7T)
          34          94       - free -  (47k)
         128  5860533000    1  freebsd-zfs  (2.7T)
  5860533128           7       - free -  (3.5k)

Thus when the zpool is created the vdev members are on a GPT device. Even if the underlying device names change the member pool names will be constant.

# zpool status
  pool: dozer
 state: ONLINE
  scan: none requested
config:

        NAME                                            STATE     READ WRITE CKSUM
        dozer                                           ONLINE       0     0     0
          raidz2-0                                      ONLINE       0     0     0
            gptid/87f5cdf0-1f60-11e4-b4e1-002590c47006  ONLINE       0     0     0
            gptid/89116305-1f60-11e4-b4e1-002590c47006  ONLINE       0     0     0
            gptid/89df42a4-1f60-11e4-b4e1-002590c47006  ONLINE       0     0     0
            gptid/8aab5a83-1f60-11e4-b4e1-002590c47006  ONLINE       0     0     0
            gptid/8b928342-1f60-11e4-b4e1-002590c47006  ONLINE       0     0     0
            gptid/8c630324-1f60-11e4-b4e1-002590c47006  ONLINE       0     0     0

errors: No known data errors

Whereas if the zpool is created with whole devices (Note: the current version of FreeNAS creates a pool with feature com.delphix:hole_birth enabled, which as of August 2014 is not widely supported by other platforms)

# zpool create -m none dozer raidz2 da2 da3 da4 da5 da6 da7

Then the status shows that the vdev is made from whole devices and not GPT slices.

# zpool status
  pool: dozer
 state: ONLINE
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        dozer       ONLINE       0     0     0
          raidz2-0  ONLINE       0     0     0
            da2     ONLINE       0     0     0
            da3     ONLINE       0     0     0
            da4     ONLINE       0     0     0
            da5     ONLINE       0     0     0
            da6     ONLINE       0     0     0
            da7     ONLINE       0     0     0

errors: No known data errors

Even though the system should not need swap, it is essential to have some swap for crash dumps. Without swap crash issues are not captured and the likelyhood of fixing them reduces.

Questions:

  1. Why put swap on the member disks?
  2. Why have swap on non-redundant disks?
  3. What's the point in having ECC RAM if memory is swapped to non-error checked swap?

 

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