Zpool

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Creating a pool

In the past, it was customary to label (i.e., partition) the disks before using them with ZFS. This is no longer the case. Now the preferred method is to have ZFS label the disks for you.

The case-sensitivity default is sensitive, but some 3rd party apps on OS X don't work entirely right if you have a case-sensitive volume. Normalization default is none, though OS X likes NFD normalization, or you may have some issues with characters.

The default for ashift is 9, but you will most likely want to create pools with an ashift of 12, which is appropriate for 4096 (4k) disks (i.e., Advanced Format disks), even if your disks are still the older 512. If you are using SSDs in your pool now, or anticipate replacing any of your pool's disks with SSDs in the future, then an ashift of 13 is a better choice. It is important to get this right now because a vdev's ashift cannot be changed after the vdev is created.

Recommended pool creation command line

sudo zpool create -f -o ashift=12 \
-O casesensitivity=insensitive \
-O normalization=formD \
$poolname mirror /dev/diskX /dev/diskY
  • Replace $poolname with the name of the pool you want. For example, "tank."
  • If you want to use the entire disk for ZFS, replace /dev/diskX and /dev/diskY with the available disks. For example, /dev/disk1 and /dev/disk2.
  • If you have already partitioned the disk, and wish to only use a slice, use /dev/diskXsN. For example, /dev/disk1s2.
  • -f instructs ZFS to label the disk for you.

Use "diskutil list" to make sure you pick the correct disk before attempting to create your pool. Note that if you partition the disk yourself, and specify a slice for ZFS, you are responsible for making sure the partition table shows the partition type is ZFS. This can be done with OS X's built-in "gpt" command or the easier to use "gdisk" command, which can be downloaded here.

You can also decide to disable access time, enable compression, and all those nice things. But that can be done at any time in ZFS. For example

sudo zfs set compression=lz4 $poolname

or for pool version 28 compatibility

sudo zfs set compression=on $poolname

If you do want compression, you should turn it on as soon as possible, or enable it at the time the pool is created. Turning compression on later will only apply to new data added to a dataset going forward, not the preexisting uncompressed data you already put in the dataset while compression was turned off.

For example, you may want to use something like this

sudo zpool create -f -o ashift=12 \
-O compression=lz4 \
-O casesensitivity=insensitive \
-O atime=off \
-O normalization=formD \
tank mirror /dev/disk3 /dev/disk4

If you absolutely must be compatible with non-OpenZFS implementations of ZFS (e.g., Oracle Solaris), you'd need to do

sudo zpool create -f -o version=28 -o ashift=12 \
-O compression=on \
-O casesensitivity=insensitive \
-O atime=off \
-O normalization=formD \
tank mirror /dev/disk3 /dev/disk4

Unplugging a pool

Pool export is not automatic in Open ZFS on OS X. Before unplugging any devices you must export all pools used by the device.

sudo zpool export $poolname

The export can sometimes fail if the mount is busy (as with all filesystems). So you might need to close any windows still accessing the ZFS volume. If you are just rebooting, you do not need to export first.