Move ZFS filesystem (including snapshots) from one pool to another

Ugly, but this would work.

Except when it doesen't;).

• Be very careful when specifying the partitions and when replacing the disks
• try it in am VM beforehand, setup the virtual disks like your hardware an dry run it 1 or 2 times.
• make a scrub before you start and take a look at the S.M.A.R.T info from the disks. You would not try this with an already flakey disk.

Important: You better have a tested backup on another medium or machine before trying it!

Yes, ZFS will grow the pool if the last 2TB disk or partition is replaced with a 4TB one ( if you have autoexpand=on for the pool )

zpool get autoexpand $pool

zpool set autoexpand=on $pool

On a sidenote: you should not use RAID-Z on disks bigger than 2TB. Your chance of getting an error on resilvering when replacing a faulted disk is very high. Please consider RAID-Z2.

These paragraphs are misleading and misinformed.

Ari has a single disk workstation. She buys a new disk and plugs it in. ZFS automatically adds the new disk space into the pool.

ZFS doesn't automatically add a newly inserted disk to a pool. You have to run the zpool command to tell how you want the disk to be used.

Her home directory is mirrored

There is mirroring only if Ari asked for but given the remaining of the sentence, that can't be the case.

while her OS and temp space is striped automatically in the background.

If mirroring has been chosen, there can be no striping. If striping has been chosen, there can be no mirroring. In the latter case, there is no "striping in the background" or "post configuration striping" with ZFS. Striping is done once at write time.

Jack has three disks of different sizes. Configuring a sensible partition and RAID set-up is completely trial and error.

Maybe, but easier than with traditional volume management.

ZFS abstracts the three disks into one pool of space, and gives the best balance of performance and security.

Triple mirroring will give both the best read performance and the best security here.

Jack declares that some media directories do not need to be fault-tolerant, and ZFS transparently stripes them across all the disks.

You cannot set properties on directories, you do it with datasets (e.g. file systems) and pools. Creating multiple file systems is a lightweight operation with ZFS though.

To summarize:

You cannot have a mirrored file system and a striped one in a single pool. All datasets belonging to a pool share the same underlying pool RAID configuration.

Assuming ditto blocks (e.g. zfs set copies=2 dataset) are what is used to provide fault tolerance, ZFS will do its best but there is no 100% guarantee all block pairs will end up to a different disks.

Even in the case all ditto blocks are physically located on different disks, the pool would not survive a whole disk failure (or more precisely, would not survive a reboot after a whole disk failure). ZFS copies will protect against limited media errors (see Andreas paragraph) but not a whole disk unavailability. Calling Ari configuration "mirroring" is misleading.

I would never advice such a configuration. Ditto blocks are a great feature if you have a single disk but still want some level of protection against partial media failure (bad blocks) or if you use deduplication on a non redundant pool. Otherwise, expectations shouldn't be set too high and either mirroring or raidz are the way to go with multiple disks pools.

Note that a combination of mirroring, raidz and striping with three different size disks is doable with ZFS by creating multiple partition based pools. For example, let say you have a 4TB, a 3TB and a 2TB disks. You can split all your disks in 1TB partitions, create a 2 TB raidz with 1+1+1 partitions, a 3 TB stripe with 1+1+1 partitions, a 1 TB mirror with 1+1 partition and a 1 TB single device pool with the remaining partition. Do not expect stellar performance with such a mixed raid setup though and strive for the applications not to access concurrently the various pools.