Encrypted Disks

Folks,

A client is asking us about data security, and keeping data encrypted on
disk. I recall there was some discussion about this before, including
things like for it to be secure an operator would need to put in a key when
the disk is mounted, that once it is mounted anyone with permissions to the
disk can read it, etc.

Given these drawbacks, I'm wondering if anyone has used an encrypted fs for
PostgreSQL and if so, what they found in terms of speed. Would this be a
big hit, or would it be pretty low impact? Also, does it cause any
potential problems with recovery from a crash, etc?

Thanks,
Peter Darley