[BBLISA] RAM for cluster net boot?

[Edward Ned Harvey]

I don't have any answer to your actual question, but I do have a comment on
going diskless.

Depending on the type of jobs you will run, you probably get better
performance if you have a disk available.  First and foremost, if your users
generate large simulation outputs or whatever, I always tell my users to
generate them under /scratch, which is always guaranteed to be local,
minimum 1.5Gb sata, and avoid the bottleneck of all machines hitting the
file server across a 1Gb connection with NFS/IP overhead.

Additionally, believe it or not, having swap space actually improves
performance.  Unless you have truly massive amounts of RAM, you can see this
easily - Run "free" and if you see that all your memory is consumed (it
usually is) then you would benefit by having swap space.  (see below)

The kernel will automatically expand itself infinitely for improved caching
and buffering, it will cache every file it reads, it will buffer every file
it writes.  It will automatically release its own memory to make room for
active processes as they request memory.  Therefore when you run "free" you
will normally see all memory consumed, (mostly by the kernel) but swap space
generally idle.  By having swap available, you give the kernel freedom to
choose which is more important to keep in memory:  The process that has been
idle, or the file which has been buffered.

The only way you can avoid this situation is to have such a massive amount
of ram that the kernel can buffer/cache every file forever, while every
active process gets all the ram it wants, and your machine never runs out of
memory.  Otherwise, you get a performance boost by allowing the kernel to
choose what's the least important thing to keep in memory.

But the avoidance of 1Gb bottleneck to network file share is the most
important reason to have a local disk.




> -----Original Message-----
> From: bblisa-bounces at bblisa.org [mailto:bblisa-bounces at bblisa.org] On
> Behalf Of Scott Ehrlich
> Sent: Wednesday, February 06, 2008 11:41 AM
> To: bblisa at bblisa.org
> Subject: [BBLISA] RAM for cluster net boot?
> 
> I'm looking at building a small cluster of disk-less 1 or 2U servers
> and will
> probably use CentOS 5.
> 
> Since these machines will not have any hard drives, what would be the
> minimum
> amount of RAM I'd need?   Also, if using Rocks or something similar,
> will that
> help cluster the RAM together so 4 servers x 4 GB RAM each = 16 GB
> available?
> 
> Some applications might be CPU intensive, others might be RAM
> intensive, so I
> need to play that balance, too.
> 
> Thanks.
> 
> Scott
> 
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