create reasonably large cache for the keys that is filled when
needed. The previous version was problematic for very large providers
(hundreds of terabytes or serval petabytes). Every terabyte of data
needs around 256kB for keys. Make the default cache limit big enough
to fit all the keys needed for 4TB providers, which will eat at most
1MB of memory.
MFC after: 2 weeks
Before this change if you wanted to suspend your laptop and be sure that your
encryption keys are safe, you had to stop all processes that use file system
stored on encrypted device, unmount the file system and detach geli provider.
This isn't very handy. If you are a lucky user of a laptop where suspend/resume
actually works with FreeBSD (I'm not!) you most likely want to suspend your
laptop, because you don't want to start everything over again when you turn
your laptop back on.
And this is where geli suspend/resume steps in. When you execute:
# geli suspend -a
geli will wait for all in-flight I/O requests, suspend new I/O requests, remove
all geli sensitive data from the kernel memory (like encryption keys) and will
wait for either 'geli resume' or 'geli detach'.
Now with no keys in memory you can suspend your laptop without stopping any
processes or unmounting any file systems.
When you resume your laptop you have to resume geli devices using 'geli resume'
command. You need to provide your passphrase, etc. again so the keys can be
restored and suspended I/O requests released.
Of course you need to remember that 'geli suspend' won't clear file system
cache and other places where data from your geli-encrypted file system might be
present. But to get rid of those stopping processes and unmounting file system
won't help either - you have to turn your laptop off. Be warned.
Also note, that suspending geli device which contains file system with geli
utility (or anything used by 'geli resume') is not very good idea, as you won't
be able to resume it - when you execute geli(8), the kernel will try to read it
and this read I/O request will be suspended.
