Swap on ZFS

[David Noel]

> the check summing of swap is quite a big deal in my opinion. You have your
> nice big server with ECC ram, but you do need to page some stuff in and
> out. Now you go and get some corruption on your disks, which you then feed
> back into your ram making the ecc irrelevant.

Now that, I think, is a very interesting point, and one I hadn't
considered. I see a great deal of value in having checksumming enabled
on a swap volume/partition. What is the default behavior when I create
a freebsd-swap partition with gpart, then hand it to the OS with
swapon? Is there any sort of error correction mechanism in place? That
alone would make a very strong case for swap on ZFS.

However, I don't see any way around the rumored issue of the system
hanging every time all my RAM is in use and I need to hit swap. Is
this still the default behavior? The wiki says yes. I suppose I could
test it myself fairly easily. But what would the solution be? If I
test it and it hangs I'd like to be able to suggest a solution in the
PR. Would adding a tunable that allowed me to reserve x MB for ZFS be
the solution? Out of curiousity, what would that value need to be set
to? How much memory does ZFS need available to write to disk?