Is it possible to mirror a ZFS pool of 4 disks with one?

Question

ZFS

There is a pool on ZFS, RAID-Z1 (I know, I know ...), consisting of four 3TB disks.
Is it possible to add a mirror to this pool, but not use a similar design of 4 3TB disks, but only one 12TB disk?

Answer 1

zpool attach [-f] pool device new_device
Attaches new_device to an existing zpool device. The existing device
cannot be part of a raidz configuration. If device is not currently
part of a mirrored configuration, device automatically transforms
into a two-way mirror of device and new_device. If device is part of
a two-way mirror, attaching new_device creates a three-way mirror,
and so on. In either case, new_device begins to resilver immediately.
-f Forces use of new_device, even if its appears to be in use.
Not all devices can be overridden in this manner.

Answer 2

As the comments say, you really can't make a vdev mirror within the same zfs pool, because that's how zfs works.
The easiest way to mirror data is to use the Send and Receive mechanism in ZFS, which consists in sending a snapshot from one pool to another, several snapshots at the same time. This process can be automated.
Everything is described in Oracle's documentation: https://docs.oracle.com/cd/E18752_01/html/819-5461...