[Beowulf] Compute Node OS on Local Disk vs. Ram Disk

Reuti reuti at staff.uni-marburg.de
Mon Sep 29 08:03:35 EDT 2008


Hi,

Am 29.09.2008 um 12:27 schrieb Alan Ward:

> Ram disks, definately. ;-)
>
> Afraid I'm still going with diskless nodes. You save
>
> 1. some money on the disks themselves
> 2. more money on solving disk failures
>
this highly depends on the used applications. Quantum chemistry code  
like Gaussian03 or Molcas would run much slower, when all  scratch  
files would be written to NFS instead of a local disk. We even use  
two or three stripped disks in the nodes.

As the disk is in the node because of our applications anyway, I also  
put the OS there.

-- Reuti

>
> 3. yet more money on cooling
>
> This may be specially important in a high-density rack situation,  
> where if you get the disks out of the way each box can get more  
> ventilators up front as well as in the rear.
>
> On the other hand, Windows Vista has had its uses(!), such as  
> driving RAM prices down as demand expands ...
>
> Cheers,
> -Alan
>
>
>
> -----Original Message-----
> From: beowulf-bounces at beowulf.org on behalf of Jon Forrest
> Sent: Mon 9/29/2008 6:44 AM
> To: Beowulf Mailing List
> Subject: [Beowulf] Compute Node OS on Local Disk vs. Ram Disk
>
> There are two philosophies on where a compute node's
> OS and basic utilities should be located:
>
> 1) On a local harddrive
>
> 2) On a RAM disk
>
> I'd like to start a discussion on the positives
> and negatives of each approach. I'll throw out
> a few.
>
> Both approaches require that a compute node "distribution"
> be maintained on the frontend machine. In both cases
> it's important to remember to make any changes to this
> distribution rather than just using "pdsh" or "tentakel"
> to dynamically modify a compute node. This is so that the
> next time the compute node boots, it gets the uptodate
> distribution. Although the mechanism for maintaining
> the distribution varies in either approach, I consider
> this a push since one mechanism isn't inherently better
> than the other.
>
> Assuming the actual OS image is the same in both cases,
> #2 clearly requires more memory than #1. There are actually
> two approaches to #2 - a) where only the OS and other stuff
> necessary to boot the system are kept in memory and everything
> else is in an NFS-mounted file system, and b) where the whole OS
> installation is kept in memory. Depending on which approach
> is taken, the RAM-based installations can take hundreds of MB
> more than a local harddrive installation. However, on a modern
> multicore compute node this might just be a few percent of the
> total RAM on the node.
>
> Long ago not installing a local harddrive saved a considerable
> about of money but this isn't true anymore. Systems that need
> to page (or swap) will require a harddrive anyway since paging
> over the network isn't fast enough so very few compute nodes
> will be running diskless.
>
> Approach #2 requires much less time when a node is installed,
> and a little less time when a node is booted.
>
> What are some of your favorite issues, positive or negative, with
> each approach?
>
> Cordially,
> --
> Jon Forrest
> Research Computing Support
> College of Chemistry
> 173 Tan Hall
> University of California Berkeley
> Berkeley, CA
> 94720-1460
> 510-643-1032
> jlforrest at berkeley.edu
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