ZFS zpool mirror drive replacement confusion

[Andriy Gapon]

on 11/08/2011 19:03 Martin Cracauer said the following:
> My computer keeps doing what I say, not what I want :-)
> 
> This zpool with a 2-drive mirror had a dead drive (ad10) which I
> removed.  After I tried to put ad2 as a replacement drive I ended up
> with the drive not in the mirror, it seems to have extended the zpool
> (it's size?).
> 
>         NAME        STATE     READ WRITE CKSUM
>         cbackup3    DEGRADED     0     0     0
>           mirror    DEGRADED     0     0     0
>             ad6     ONLINE       0     0     0
>             ad10    REMOVED      0     0     0
>           ad2       ONLINE       0     0    11
> 
> Unfortunately I didn't keep note of the specific command that I used.
> 
> Two questions:
> 
> - Does somebody know offhand what I did and what I should have done
>   instead to get ad2 into the mirror?

You confused 'add' and 'attach' commands or something like that.

> - Would there have been a way to remove ad2 from this set? 

No.

> It didn't
>   let me, saying it can only remove drives from mirror or raid sets.
>   I think it should have been possible before I mounted the filesystem
>   in there read-write for the first time after the drive add.  This
>   seems like an easy mistake to make.

This has been reported to the ZFS people (upstream) many times, still they didn't
add any safety nets.

> The array doesn't contain important data, just trying to sort out
> things for the future.

Read the docs, twice, be careful, take backups before any modifying actions.

-- 
Andriy Gapon