[Beowulf] Why do we need SANs

Mark Hahn hahn at mcmaster.ca
Thu Mar 3 21:15:16 PST 2011

> This is a thought provoking article on storage:
> http://www.theregister.co.uk/2011/03/01/why_do_we_need_sans/

I didn't think it was particularly good: basically just the observation
that you can put lots of servers instances into a big box and do away 
with a lot of infrastructure.  call it "mainframe".

the reason this is not good and maybe even bad is that it pretends 
that there's no need for scalable systems.  and it seems to be based 
on the premise that infrastructure such as FC, which has always been
very complex and expensive, would somehow be necessary.

besides, who would even propose SANs these days?  they're inherently
hostile to sharing, and it's all about sharing today...

> In the light of the recent thread on Thunderbolt, note:
> "Infiniband or ..... extermalised PCIe bus"

as far as I can tell, thunderbolt doesn't introduce anything interesting
regarding addressing, switching fabrics or low-latency transactions between
hosts.  I can't tell whether Intel wants to push it in that direction,
or whether it's a short-term sop for Apple.

> Do we see the HPC system of the future being a huge blade server farm,
> running lots of virtual machines,

hell no.  first of all, blades are done.  they never made much sense,
and what little sense they had (higher efficiency PSUs and cooling)
has now become available in standard/commodity parts.

but more importantly, why the heck would we want VMs for HPC?  virtualization
is great for packing lots of different, low-duty-cycle server instances 
onto less hardware.  and it provides some help in shuffling them around.
HPC is high duty-cycle, and doesn't the inherent inefficiency of virt.
being able to move processes around would he helpful for HPC, but it doesn't
need to be whole OS instances.

> with some sort of object storage system spread across the machine
> running on selected blades?

SANs simply lack the expressive power necessary to support modern 
file/object/database/web sharing protocols.  and the SAN world, which 
considers 8Gb FC to be sexy, has been left in the dust by 40 Gb IB 
or even ethernet.  (if IB had built on ethernet protocols rather than 
badly reinventing the wheel, no one would even remember FC today...)

regards, mark hahn.

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