Difference between revisions of "Zpool"

Revision as of 02:34, 4 March 2016

Creating a pool

In the past, you would label (i.e., partition) disks yourself before using them with ZFS. This is no longer the case. Now, in most cases, you will instead want to have ZFS partition disks for you.

On OS X you will want to use NFD normalization, or quite a few applications won't work (e.g., iTunes).

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 diskX diskY

Or using "\" to show it broken up over multiple lines:

sudo zpool create -f -o ashift=12 \
-O casesensitivity=insensitive \
-O normalization=formD \
$poolname mirror diskX 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 diskX and diskY with the available disks. For example, disk1 and disk2.
• If you have already partitioned the disk, and wish to only use a slice, use diskXsN. For example, disk1s2.
• -f instructs ZFS to label the disk for you.
• -o ashift=12 instructs ZFS to use 4K sectors, as described here: http://wiki.illumos.org/display/illumos/ZFS+and+Advanced+Format+disks

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. For detailed instructions, see 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 disk3 disk4

Or using "\" to show it broken up over multiple lines:

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

Or using "\" to show it broken up over multiple lines:

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

Unplugging a pool

The equivalent of unmounting a device before unplugging it is exporting a pool in ZFS terminology.

Before unplugging any devices you must export all pools using 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 applications still accessing the pool. Also, there is an option "-f" to force unmount while exporting:

sudo zpool export -f $poolname

If you are just rebooting, you do not need to export first.

Feature flags

If you are creating a pool on another OpenZFS platform, and you want to make sure that pool is compatible with OpenZFS on OS X, you must not enable any feature flags not yet supported by OpenZFS on OS X.

For instance, let's say you were creating a pool using OmniOS, which currently supports more feature flags than OpenZFS on OS X.

To start out with all feature flags disabled, use the -d option when creating the pool:

sudo zpool create -d $poolname c2t1d0

To selectively enable only the feature flags supported by OpenZFS on OS X (as of 2014-08-01) use -d combined with -o feature@..., as follows:

sudo zpool create -d -o feature@async_destroy=enabled -o feature@empty_bpobj=enabled -o feature@lz4_compress=enabled $poolname c2t1d0

Or using "\" to show it broken up over multiple lines:

sudo zpool create -d \
-o feature@async_destroy=enabled \
-o feature@empty_bpobj=enabled \
-o feature@lz4_compress=enabled \
$poolname c2t1d0

And don't forget to specify the other options you want, as discussed above:

sudo zpool create -d \
-o feature@async_destroy=enabled \
-o feature@empty_bpobj=enabled \
-o feature@lz4_compress=enabled \
-O compression=lz4 \
-O casesensitivity=insensitive \
-O atime=off \
-O normalization=formD \
$poolname c2t1d0

(Note that illumos distributions such as OmniOS handle ashift indirectly as described here: http://wiki.illumos.org/display/illumos/ZFS+and+Advanced+Format+disks)