Re: huge box of ram?
- From: Paul Rubin <http://phr.cx@xxxxxxxxxxxxxx>
- Date: 28 Apr 2006 12:14:42 -0700
lr@xxxxxxxxx (Ralph Becker-Szendy) writes:
Is that a big deal? What is so high tech about this? It's just a
bunch of dimm sockets on a board, right?
That's the easy part. The hard part is the ECC, the memory
controller, the memory bus to interface controller (typically a
high-end CPU in and of itself), and the redundancy of the interface
controller. You are talking about a big RAM-disk. The easiest way to
acquire this is to buy a disk array with a lot of cache RAM.
Well, fine, what about the same thing without the disks?
Give me $50M, and I'll found a startup that will manufacture such
large RAM-disks with Infiniband interfaces. You have to commit to buy
$200M worth of them per year from me though - because there seems to
be little other demand for them.
Are you joking? This thing doesn't need any ASIC's. It's all off the
shelf.
Find a motherboard with a two dozen PCI slots (which is nearly an
electrical impossibility, you will have multiple PCI busses at this
point, which is no longer a commodity PC).
32 sockets (as mentioned above) would take four PCI cards with 8
sockets per card. Nothing like dozens of PCI slots. Is there some
obstacle to an 8-dimm version of the Giga-byte card, that handles
2gb dimms? The existing card takes 4 dimms with 1gb each.
If you want to use the RAM as adressable core, you need a CPU with a
huge address space, which is not available on PCI slots. This just
can't be done with commodity hardware.
64-bit PCI doesn't handle 64-bit addresses? Hmm.
What PC's exist that can take more than 16GB? I'd have to use dozens
of them to get 500 GB of ram.
Go to HPs website, and look for whatever the Superdome is called now.
Look for an Itanium-based machine with 64 or 128 CPUs. I bet that
thing can take 500 GB of RAM.
Hmm, the 128-way version can take 1TB of memory. But why does that
ram capacity only exist on monstrous processors like that (the 32-way
version was $600-700K in Nov 2001, which by Moore's Law would put the
128-way version at around $300K today)? That's what I don't
understand. I can put almost infinite amounts of commodity disk space
on a consumer PC. I don't understand why there's not comparable
ramdisks.
What's the speed of CF or SD? You might wish for a spinning harddisk
instead. Also, Flash memory (like SD and CF) can only be overwritten
so many times (the estimates range from 10K to 10M times); not
suitable for use like RAM. And the reliability of such a device is
likely to be horrible.
The write wear may be an issue though most of the traffic is read.
The application takes many thousands of small, random-access requests
per second, making disks not such a great solution.
For example, used 9, 18 or 36GB SCSI or FC disks, with 15K RPM (nice
Seagates), can be had used for not very much money today. Those have
very very good seek times, much better than typical SATA disks. If
you can stripe your workload over a hundred drives, and short-stroke
them (only the first few GB of each drive are used), you might be in
business.
This is maybe worth thinking about, just for the added parallelism.
.
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