zfs over geli over zfs (was: Re: zfs flag denoting unclean shutdown?)

Nikos Vassiliadis nvass at gmx.com
Thu Oct 3 16:20:52 UTC 2013

On 10/02/2013 08:13 PM, Matthew Seaman wrote:
> On 02/10/2013 16:34, Nikos Vassiliadis wrote:
>> Is there a way to know if a zfs pool had an unclean shutdown?
>> An attribute or maybe something during mount time similar to what ufs
>> does (WARNING: / was not properly dismounted)?
> Other than looking at the system logs for evidence of an abnormal
> shutdown, no.  (Absence of anything in the logs is pretty good evidence
> for the system falling over pretty hard... Usually something to do with
> the power being turned off.)
> However, due to the design of ZFS unclean shutdowns like this are
> nowhere near as problematic as on UFS.  Basically, you're guaranteed
> that what is written on disk is always consistent.  You might lose a few
> transactions -- essentially the last few seconds of file system activity
> -- but that doesn't usually make a great deal of difference after the
> system reboots again.  Oh, yeah -- absolutely no time will be needed to
> be spent cleaning and repairing filesystems: with ZFS, reboot after
> crash is as fast as a normal reboot.

Thanks Matthew, I realized I should have used a more appropriate 
subject. I'll explain what my actual goal is:)

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;)

The obvious solution is to create two slices and dedicate a slice to 
geli. Like this:
mypool lives on slice1
myencpool lives on slice2.eli

I am after this:
mypool lives on slice1
myencpool lives on /dev/zvol/mypool/avolume.eli

The second scheme will allow me to have an encypted home and not to 
pre-allocate space. A quick test showed that it might work... On the 
other hand conceptually seems like a very bad idea haha.

I think I've heard people doing this zfs over geli over zfs before...

Thanks for any thoughts, Nikos

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