[K12OSN] Server Quest

[odonovan]

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---------- Original Message -----------
From: Ken Johnson <ken.tech1 at gmail.com>
To: K12LTSP <k12osn at redhat.com>
Sent: Fri, 25 Mar 2005 14:29:13 -0500
Subject: [K12OSN] Server Quest

> I am looking for a server to support a 10 node K12LTSP network (that
> may be larger in the future).
> 
> Since budget is a consideration, I am looking on EBay for a
> "previously loved" Dell Server.
> 
> I plan on at least a gig of RAM, 2 If I can afford it. (is this 
> enough?)

RAM is the main determinant of the number of users a system can support.
RAM usage largely depends on the type of user you have.  OOo doing simple WP
tasks takes a lot less RAM than multimedia presentations; Firefox on simple
static sites can demand in the order of 20 - 30MB/instance but with tabbed
browsing and multiple jvm invocations that can go over 100MB/user.  We budget
between 50 and 100MB /concurrent user for both of these apps.  The fewer users
you have, the more variability you can have.  1G is likely okay for the 10
users you as long as they don't all do something really demanding at once.
100MB/concurrent user is a conservative figure; it allows at least 40 users
(and often >50 depending on usage patterns) on 4GB RAM.

Proc speed becomes an issue if you have large numbers of "kiddie-type" users
doing graphics intensive activities (graphics-based typing activities,
elementary school animations, java-based interactive graphics).  Here 20 to 30
users can cause a dual xeon to begin to look aenemic (even with a Gig enet
server connection to a fully switched client network). I use a rule of thumb
of 100MHz per concurrent user as being more than adequate on larger systems.
(ie a dual 3GHz machine should support about 60 users with some room to spare)

>From a systems point of view a dual proc system is a minimum requirement. 
Many well behaved daemon applications like Apache, Sendmail ... can be run on
a single proc machine because they are well behaved.  
Applications that have interactive user interfaces
OFTEN break and result in runaway process that consume all CPU resources
(typically on 1 CPU).  Even with automated kill scripts (one is available on
the LTSP site) or custom-made programs,  administrator monitoring and
intervention is needed.  Multiple procs cut the need for intervention as the
system degenerates in a nice linear orderly way.  Then you can simply automate
dumping all user process in the middle of the night and know your server is
ready for the next day.


> 
> I think I need SCSI (am I correct?) I have seen some servers that 
> have SCSI drives ranging in size from 9 to 36 gig. Would 9 be 
> enough? Seems a little small.

If you are implementing  a simple system (with the home directory files on the
server), I would recommend at least 36G drive space (even on a 10 user
system).  That allows for about 10G for the OS and applications (A full
K12LTSP install needs at least this much.) and the rest
for home. 

> 
> I have never worked with SCSI drives.What pifalls are there? 

IMHO none. But you do need to know a little about scsi configurations (hot
swap/SCA bays --common in rackmount systems -- really simplifies the setup).

>Budget
> does not permit making a mistake, I need to get a server that will
> work with K!2LTSP.
> 
I have worked with simple IDE systems for up to 20 users and quickly moved to
SCSI for general use.  I have used
RAIDed IDE for systems that have had up to 5 users and they work well (but I
wouldn't trust the configuration for larger sets of users where home
directories were stored on the server. 

If you must used ATA/IDE be sure to put each disk on a separate bus.  SATA is
a good compromise. The most cost effective system today seems to be a low cost
SATA software RAID which can be set up from the K12LTSP install (or hw if the
controller supports it). Your server has to provide SATA or it can be added
economically (be sure the controller is one known by Fedora).

If the server has a SCSI controller on board make use of it.  The difference
in price on the disks is well worth it.  

> Some have 3 drives and RAID- will this work with k12LTSP? Do I need this?
> 
Should they be RAIDED?  If the system is storing files for anyone other than
yourself, you should provide some level of redundancy/protection.  Otherwise
you have to go to backup whenever anything happens and that represents a lot
of time and effort for you (and downtime for the clients). On a small system a
simple hardware or software RAID 1 (mirror) is the most economical.

> Some require rack mounting (never done that either)- should I avoid
> those? 
Not necessarily, they have a number of attractive qualities and are the only
way to go if you have multiple servers.  But two characteristics of rack
systems work against some of the criteria you have set out.  Components are
generally very proprietary (If a power supply goes, where do you get a
replacement quickly). and they tend to be somewhat pricer that conventional
boxes. But the biggest problem is likely to be ...

>The server will be placed in the same room as the 
> workstations. 

Rackmount systems generally pay little or no attention to noise attenuation
and you probably don't want to have one of those beasts in your work area!

Do I have to mount it on a rack? 
No but they tend to take up a lot of surface space unless it can be mounted
vertically somethere. Got a small closet nearby?

(Budget and Space are 
> considerations.)

Yes, yes, yes!

Good luck!!

oo
/
-



> 
> Any advice would be appreciated.
> 
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------- End of Original Message -------