A brief introduction to ZFS (disk) GUIDs
Although ZFS commands like '
zpool status' will generally not tell you
this, ZFS does not actually identify pool devices by their device name.
Well, mostly it doesn't. The real situation is somewhat complex.
ZFS likes identifying pool-related objects with what it calls a 'GUID',
a random and theoretically unique 64-bit identifier. Pools have GUIDs,
vdevs have GUIDs, and, specifically, disks (in pool configurations) have
GUIDs. ZFS internally uses the GUID for most operations; for instance,
almost all of the kernel interfaces that
zpool uses actually take
GUIDs to identify what to change, instead of device names.
(The 'numeric identifier' that you can use to have '
import a pool is the pool's GUID.)
In a pool's configuration, entries for disks have a bunch of information
to help ZFS identify the right device: the GUID of the disk, the device
it's expected to be found on, and the physical path and device ID of
that device. You can see most of a pool's raw configuration, complete
with this information about each disk, with '
zdb -C <pool>'.
zdb doesn't print information about spare disks,
only about disks that are in vdevs.)
As you might guess, disks being used by ZFS have an on-disk label (in
fact they have four copies of it, two at the start of the ZFS slice and
two at the end). Among other things, this disk label has the disk's
GUID. You can dump a disk's ZFS label with '
zdb -l' on the ZFS slice
(normally slice 0, '
s0', if you have given ZFS the full disk).
(On disks that are part of a live vdev, the disk label also has a copy of the vdev's information; on spare disks, all the label has is the disk's GUID, the version, and the state.)
zpool import <pool>' works by finding a copy of the
nominal full pool configuration and then searching all of the disks to
find the disk GUIDs mentioned in the pool configuration. If the system
can find enough of the disks, it can actually import and bring up the
(I'm not entirely clear where the full pool configuration is stored;
it's in the pool somewhere, but it's unfortunately not in the disk
labels, so it's not trivial to dump it with
Note that ZFS GUIDs are not real GUIDs. Real GUIDs are 128-bit objects and are conventionally printed in a special format; ZFS GUIDs are only 64-bit ones and are conventionally printed as plain decimal numbers.
zdb versus the full pool configuration
What seems to be going on with '
zdb -C' is that it dumps out the
in-kernel pool configuration nvlist, and while
the kernel keeps track of spares and other things, it does not keep them
in the in-kernel pool config nvlist; instead it stuffs them into other
bits of data structures that
zdb does not print out.
Things like '
zpool status' look at the full pool configuration, but
they don't print the raw nvlist; instead they helpfully process it for
you and hide various bits of what is going on.
(I wound up writing a full pool config nvlist dumper myself; you can get a copy of the current source code here.)