On 2013-01-22 23:32, Nico Williams wrote:
IIRC dump is special.

As for swap... really, you don't want to swap.  If you're swapping you
have problems.  Any swap space you have is to help you detect those
problems and correct them before apps start getting ENOMEM.  There
*are* exceptions to this, such as Varnish.  For Varnish and any other
apps like it I'd dedicate an entire flash drive to it, no ZFS, no

I know of this stance, and in general you're right. But... ;)

Sometimes, there are once-in-a-longtime tasks that might require
enormous virtual memory that you wouldn't normally provision
proper hardware for (RAM, SSD) and/or cases when you have to run
similarly greedy tasks on hardware with limited specs (i.e. home
PC capped at 8GB RAM). As an example I might think of a ZDB walk
taking about 35-40GB VM on my box. This is not something I do
every month, but when I do - I need it to complete regardless
that I have 5 times less RAM on that box (and kernel's equivalent
of that walk fails with scanrate hell because it can't swap, btw).

On another hand, there are tasks like VirtualBox which "require"
swap to be configured in amounts equivalent to VM RAM size, but
don't really swap (most of the time). Setting aside SSDs for this
task might be too expensive, if they are never to be used in real

But this point is more of a task for swap device tiering (like
with Linux swap priorities), as I proposed earlier last year...


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