zfs on geli vs. geli on zfs (via zvol)

[Todd Wasson]

Thanks to both C. P. and Pete for your responses.  Comments inline:

> Case 1.) is probably harmless, because geli would return a
> corrupted sectors' content to zfs... which zfs will likely detect
> because it wouldn't checksum correctly. So zfs will correct it
> out of redundant storage, and write it back through a new
> encryption. BE CAREFUL: don't enable hmac integrity checks
> in geli, as that would prevent geli from returning corrupted
> data and would result in hangs!

Perhaps the hmac integrity checks were related to the lack of reporting of problems back to zfs that Pete referred to?  Maybe we need someone with more technical experience with the filesystem / disk access infrastructure to weigh in, but it still doesn't seem clear to me what the best option is.

> Case 2.) is a bigger problem. If a sector containing vital
> geli metadata (perhaps portions of keys?) gets corrupted,
> and geli had no way to detect and/or correct this (e.g. by
> using redundant sectors on the same .eli volume!), the whole
> .eli, or maybe some stripes out of it, could become useless.
> ZFS couldn't repair this at all... at least not automatically.
> You'll have to MANUALLY reformat the failed .eli device, and
> resilver it from zfs redundant storage later.

This is precisely the kind of thing that made me think about putting zfs directly on the disks instead of geli...  This, and other unknown issues that could crop up and are out of geli's ability to guard against.

> There may be other failure modes involved as well. I don't know.
> But in most practical day to day uses, with enough redundancy
> and regular backups, a zfs-over-geli should be good enough.

I understand the point here, but I'm specifically thinking about my backup server.  As I understand it, part of the purpose of zfs is to be reliable enough to run on a backup server itself, given some redundancy as you say.  Perhaps asking for encryption as well is asking too much (at least, unless zfs v30 with zfs-crypto ever gets open-sourced and ported) but I'd really like to maintain zfs' stability while also having an option for encryption.

> I wouldn't put {zfs,ufs}-over-geli-over-raw-zpool though, as this
> would involve considerable overhead, IMHO. In this case, I'd
> rather use a gmirror as a backend, as in a setup:
>  {zfs,ufs}-over-geli-over-{gmirror,graid3}
> or something similar. But I've never tried this though.

I understand about the overhead, but I'm interested in using zfs via a zraid to avoid using gmirror or graid3, because of the benefits (detection of silent corruption, etc.) that you get with a zraid.  I think your suggestion is a pretty good one in terms of performance/reliability tradeoff, though.  In my specific case I'm more likely to pay a performance cost instead of a reliability cost, but only because my server spends most of its time hanging around idling, and throughput isn't really an issue.  Thanks regardless, though.


Todd