Scrub on exported backup pool

I have a raidz backup pool in an four disk external case connected to my Nas.

I Import it right before the backup and export it afterwards. As it is an usb enclosure I don’t really want to have it imported all the time.

But how can I do monthly scrubs on that pool?

Currently I import it before the scrub, start the scrub and just wait until the backup which runs the next morning exports it again.

Is there a better way to do that?

Not that I know of. You can’t scrub a pool unless it’s imported.
If you’re worried about reliability, I would recommend switching it to eSATA with a Probox enclosure or move the whole thing to SAS.


Or, better yet, grab a cheap computer to put the backup pool in. Basically any retired desktop-class (or laptop-class) machine with 4GiB RAM or more and enough connectivity to plug the drives in.

Set it up for automated backup over the network, leave it powered on all the time, monitor it to make sure it continues to get new snapshots replicated in. Easy peasy. Make sure the same login you use on your main box WON’T get you into the backup box, of course. Or if you’re extra paranoid, set up a firewall on it that refuses all incoming connections, period… which leaves it capable of backing up your stuff, but now requires you to go get physically in front of it if you want to mess with stuff.


I have a similar setup with 3 different USB3 enclosures that I rotate between live backup, cold local backup, and cold offsite backup every couple of weeks. I basically do the same thing, except that I fire a small script that calls zpool status and scrapes the output once every 10 minutes that way it’s only ever mounted when actually backing up to it and when scrubbing it. I’d consider having another computer to back up to, but prefer the lower power alternative that I currently have simply because it’s easier to maintain the local cold backup and cold offsite backup and there’s only ever one enclosure plugged in at a time. That and USB3 is still way faster than ethernet unless you spend a lot of money to go 5G or faster network.

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I did it more-or-less the same way.

Not directly related to scrubbing, but I had a very similar setup as you with an external USB3-SATA dock, and the electronics in the dock died after 3 years due to dried-out electrolytic capacitors (it was readily apparent from the bulging on both ends of several of the power supply caps).

These were rated for 105C, so they weren’t even (supposed to be) crappy under-dimensioned capacitors. Even after replacing them with good caps, I couldn’t make the power rails work correctly, so I ended up paying for a brand new dock.

Apparently the docks are only designed for intermittent use and not running 24h a day.


that’s good to know!

IME those docks are designed to get you to buy them, and that’s where the design stops. I’ve tried so many of those things over the decades, and they never worked reliably for very long even WITH intermittent use. I just wanted them to be handy ways to temporarily connect random drives, too!

These days I use a server with real SATA hotswap bays for that purpose. In some ways it seems like ridiculous overkill… But it’s reliable, where none of the two to four bay docks ever were.

as i use a sas controller anyway i could buy a disk shelve. but this seems to be a little overkill to me (it’s still a home nas) and i don’t want something rack mounted.
same for an additional backup server. just too much for my needs.

Yeah, that was my thinking as well - also considering the power consumption of a full system. Mine is just an rpi4 + 2-slot dock, which should be close to a minimum power solution.

Sad that the docks are so poorly designed. I’m gonna swap out my nas, and maybe the old Avoton mobo could take over the duty as onsite backup rig.

@kaihp you can pick up a $75 odroid system with similar power consumption to your pi, but proper full SATA connectivity.

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Well yes, if I had been in the USA.

Googling says it’s around €160 including the PSU, box (HC4-P kit) and shipping in the EU.

The power of the Odroid appear to be similar to the Avoton I already have, so even electricity cost appears not to be a compelling argument for me, in my situation.