After having read the section about
zpool export in
man zpool, I am worried somehow:
Exports the given pools from the system. All devices are marked as exported, but are still considered in use by other subsystems. The devices can be moved between systems (even those of different endianness) and imported as long as a sufficient number of devices are present.
What does "but are still considered in use by other subsystems" mean? And what does "as long as a sufficient number of devices are present" mean?
I have a fairly complex backup script which replicates VM storage on a production server to a standby server, based on ZFS snapshots (to be precise, it is a system of various scripts on the host and in the VMs which work together to freeze the file systems in the VMs, take the ZFS snapshots, thaw the filesystems in the VMs, and replicate the snapshots to the standby server). This part is working like a charm.
But actually, I need a sort of backup in addition to the replication. That is, I would like to backup the VM storage onto a disk which I can afterwards remove from the box where it is attached to, and store it safely at another location.
I have thought thoroughly about the best method and have come to the following idea:
- In the standby server, mount a SATA / SAS HDD tray so that I can plug and unplug HDDs there (or use one of its existing trays which are connected to its backplane).
- At the standby server, plugin a new HDD and make a new ZPOOL, consisting of only one VDEV which consists only of that HDD.
- Let the production server replicate its VM storage to that new ZPOOL in addition to the ZPOOL to which it replicates already.
- After the replication, export the new ZPOOL, unplug the new HDD, and store it at a safe place. **
- At the standby server, plug in a second new HDD which also represents a ZPOOL, and repeat the previous two steps. ****
- And so on ... (for example, daily replace the HDD currently attached to the standby server by the other one, or use 7 HDDs and rotate them on a weekly basis, etc.).
** This is the step which I am not comfortable with (after having read the section cited above). On one hand, it should be no problem to remove the HDD after the ZPOOL (whose only device it is) has been exported, because file systems get dismounted (and hence flushed) before the export. On the other hand, the manual says that this HDD will still be "considered in use by other subsystems" even after exporting the pool, which makes me believe that it is a bad idea to simply remove it in this situation.
Therefore, I would like to know what this statement exactly means, and how I can make the HDD being considered not in use any more by other subsystems.
**** I am aware that I'll have to put some effort into this. The plan is to create a script along with an appropriate
udev rule to have the standby server recognize when the HDD is plugged in and to import the existing ZPOOL (the HDD will be connected directly via SATA-3 or SAS 12G, not via USB). But this is not part of this question.
What exactly does
zpool export do, or, in other words, what steps do I need to perform before I can safely physically remove a HDD which is the only device in a ZPOOL after that ZPOOL has been exported?