[Beowulf] ram disk - diskless nodes

Mark Hahn hahn at physics.mcmaster.ca
Fri Jul 7 00:01:39 EDT 2006


> couldn't you boot from a usb storage device, rather than use up valuble
> ram?

as mentioned before, the memory overhead of net-booting can be quite 
trivial.  a 96x2 cluster I did some years ago has 4-8GB nodes, and uses
at most a few MB of initrd when booting (freed when the NFS root can 
be mounted.)  unless you've got some pretty specialized hardware,
you're not going to build a cluster with less than 1GB/node these days.
as such, even tens of MB of ramdisk is not any kind of big deal.

booting from usb is really just another diskful solution.  they're fine,
and usb has some clear advantages over moving parts.  but it's still
something you have to configure for each node, and therefore less malleable.

> shurely is cost effective as 512 of ram cost about 3 times as much
> as 512 of usb mem.

you're right, I hadn't thought of it in those terms.  but it's still
per-node-installation, and therefore less scalable in many ways.
bear in mind that when people opt for diskful solutions, they usually
want big local storage for swap, /tmp, etc.  I claim that net booting
_and_ diskful is the right thing (just don't boot from the disk).
after all, vendors will often charge _extra_ for diskless nodes, since 
their norm is diskful.  as part of a $1-4K node, evne a 160G disk 
is just not a major concern.

> and you could pull the usb 'keys' out to re configure or what not from
> other nodes that the admin can work from.

you don't want to do that for even a 64-node cluster, let alone hundreds...

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