best way to backup mirror with 3rd disk?

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best way to backup mirror with 3rd disk?

Postby kgreene » Thu Sep 07, 2017 1:36 pm

I'm planning on doing a backup of a 2 disk mirror before doing an upgrade on the pool (an old maczfs pool, I posted about this recently in the non openzfs pools subforum.) I'm running 1.6.1.

The current mirror is two 4tb drives. I have a 3rd 4tb drive so I was thinking I could just attach it to the pool and wait for it to resilver, then remove it from the pool before upgrading. If the upgrade fails horribly I can use my new 3rd drive copy to immediately reconstruct the original rather than having to copy from other format backups (I have files on a western digital mycloud mirror which I'm not that happy with).

Anyway, the question is, what is the best way to do this, or is there something bad or flawed with this idea? I've found other zfs discussions online elsewhere about this and they suggested attaching and then splitting the new disk off. I was originally thinking of either attaching, resilvering and detaching or attaching a spare, resilvering and removing it. I would like input about the os x version rather than another fork... :)

Which of these would work or would be best?

After I get a successful pool upgrade (I will trust a scrub unless there is a better idea) I will reuse this disk in a new mirror.
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Re: best way to backup mirror with 3rd disk?

Postby xbytor » Thu Sep 07, 2017 2:41 pm

>attaching, resilvering and detaching

I've done this (or something similar) several times over the years as I've upgraded my drives with no problems.
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Re: best way to backup mirror with 3rd disk?

Postby lundman » Thu Sep 07, 2017 5:20 pm

I could just attach it to the pool and wait for it to resilver, then remove it from the pool before upgrading.


Depends on what you mean here, if I remember correctly, if you issue "zpool detach pool backupdisk" it will mark "backupdisk" as empty, and not containing any data (even though it does). Whereas, just unplugging it from the system (offline if you want, or unplug at reboot) and it will be a working backup of the pool. This is usually how I do my work.

Then,only at the end, if the update went well, would I issue "zpool detach" to remove the redundant vdev.
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Re: best way to backup mirror with 3rd disk?

Postby kgreene » Thu Sep 07, 2017 8:56 pm

OK, great. Yeah, I want the disk to be a viable backup so maybe that's one way. It looks like split might be an option as well (at least the command is listed, I've never used it).

It'll take a while to resilver anyway so I can start that shortly...

thanks
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Re: best way to backup mirror with 3rd disk?

Postby stumble » Fri Sep 08, 2017 2:55 pm

I've used split to do this.
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