zfs over geli over zfs

Matthew Seaman matthew at FreeBSD.org
Thu Oct 3 21:16:17 UTC 2013

On 03/10/2013 17:20, Nikos Vassiliadis wrote:
> I am after a really specific use-case and the last minute transactions
> are important. Using a zpool over geli over a zvol. I'd like to know if
> during shutdown the kernel flushes all zfs files caches in order so
> these last minutes transactions won't be lost. The unmounting order is
> far from obvious (zfs over geli over zfs) and i wonder if such a scheme
> will succeed. I can't afford losing the last transactions of my home dir
> every time i shutdown my laptop;)

If it's a normal clean shutdown, then yes, all pending transactions will
be committed to persistent storage.

Normally you'ld do something like this by creating geli devices on disk
partitions (usually via gpt nowadays), and then creating your zpool from
those geli devices.  (Typically you'ld just use one geli device in your
zvol, which doesn't offer any resilience but avoids potential
cryptographical fubars like having two crypttexts known to come from the
same plaintext: something that can make it considerably easier to break
the encryption.

Using a zfs exported as a raw device layered with geli is a good way to
get round that, but I think you're probably better off creating a
standard UFS on top of the geli partition, rather than creating a second
layer of zpool and zfses.

(I don't actually know: this is just me guessing without ever having
tried this in practice.  I'll willingly cede to anyone with actual
experience of this sort of thing.)



Dr Matthew J Seaman MA, D.Phil.
PGP: http://www.infracaninophile.co.uk/pgpkey

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