When are diskless compute nodes inappropriate?

Bill Broadley bill at math.ucdavis.edu
Thu Jul 17 16:48:38 EDT 2003

On Tue, Jul 15, 2003 at 11:48:05AM -0400, Tod Hagan wrote:
> Okay, I'm convinced by the arguments in favor of diskless compute
> nodes, including cost savings applicable elsewhere, reduced power
> consumption

5-10 watts.

>, and increased reliability through the elimination of
> moving parts.

Indeed.  Although similar reliability can be had if you can survive
a disk failure.

> With all the arguments against disks, what are the arguments in favor
> of diskful compute nodes? In particular, what are the situations or

Swap, and high speed disk I/O.  35 MB/sec of sequential I/O to a local disk
is very hard to centralize.  If you can make do with much less then it's not
to much of a big deal.

For our 32 node cluster on boot we:
	netboot a kernel
	kernel loads a ramdisk
	disk is partitioned
	disk is mkswaped
	/scratch and /swap are mounted.

So this leave ZERO state on the hard disk, so if a disk dies just reboot
and the node works (but doesn't have /swap and /scratch), if you pull
a disk off a shelf and stick it in a node you just reboot.

Very nice to minimize the administrative costs of managing, patching,
backing up, troubleshooting etc of N nodes, with possibly different images,
and of course any state.

My central fileserver is a dual-p4, dual PC1600 memory bus, 133 Mhz/64 bit
PCI, and several U160 channels full of 5 disks each.  I see 200-300 MB/sec
sustained for large sequential file reads/writes.  Granted the central
fileserver can not keep up with 32 nodes wanting to read/write at 35 MB/sec,
but it's enough to usually not be a bottlneck.

Bill Broadley
UC Davis
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