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

[C. P. Ghost]

On Tue, Jun 28, 2011 at 8:45 PM, Todd Wasson <tsw5 at duke.edu> wrote:
> While zfs on geli is less complex (in the sense that geli on zfs involves
> two layers of filesystems), I'm concerned as to whether encrypting the
> device will somehow affect zfs' ability to detect silent corruption,
> self-heal, or in any other way adversely affect zfs' functionality.  In my
> mind, if I use geli on zfs, then I've got zfs directly on a device and the
> zvol it's providing will be transparently gaining the benefits of zfs'
> various features, providing a "safety layer" against device failure and
> silent corruption that I'm not sure if geli would detect.

I'm going out on a limb here, but that's how I see it without intimate
knowledge of the geom/geli and zfs code paths involved.

Basically, you leave the transparent encryption of sectors to geli,
and the integrity checking and repairing to zfs, and you should
be *mostly* secure, except for the failure modes below.

.eli devices behave just like normal disks, in the sense that
they are a block device that transparently encrypts and decrypts
sectors when they are accessed. So what could go wrong
there:

  1. single sectors may be corrupted on disk (e.g. bits flipping).
  2. geli metadata (keys etc...) are destroyed. Same for glabel metadata.

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!

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.

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 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.

-cpghost.

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