Setup used in Greek Schools

[Job Cacka]

Years ago, we used 64MB of RAM per thin client connection as a rule of thumb for calculating RAM. So if your class size is 25 thin clients you would want 64 MB * 25 = 1600 MB or 1.56 GB. I think this rule of thumb is old however. We were using LTSP 5 on Ubuntu 10.04 I believe. However if you doubled the RAM requirements you would still be under your 4GB. 

 

Have fun!

  Job Cacka

 

From: edubuntu-users-bounces at lists.ubuntu.com [mailto:edubuntu-users-bounces at lists.ubuntu.com] On Behalf Of David Groos
Sent: Thursday, June 11, 2015 6:54 PM
To: Veli-Matti Lintu; Edubuntu Users Group
Subject: Re: Setup used in Greek Schools

 

Thanks all for weighing in. I didn't know about the role of caching for root file system (i.e. /) in LTSP but it makes complete sense why /home should get the ssd love not the root which I had supposed. I'm now questioning if the 4 gigs of RAM that I used for the "teacher computer" in my classroom was enough. Any sense of amount of RAM that would mostly avoid this bottleneck? Let's say one was running 14.04 Ubuntu.

 

The dm-cache sounds the most eloquent solution but is beyond my tech knowledge/resources so using the SSD for /home seems like the way to go. If I can't get any SSD's, is there any reason to use 2 standard HD's in some format such as a RAID or dividing off the /home partition? Our teacher computers will be i5's with space for at least 2 HD's.

BTW, here's a nice 2014 TEDx presentation on Libre software by Richard Stallman--probably some have already seen it: http://teemuleinonen.fi/2015/06/09/why-freelibreopen-source-in-learning-is-important/.

 

 

On Thu, Jun 11, 2015 at 2:20 AM Veli-Matti Lintu <veli-matti.lintu at opinsys.fi> wrote:

2015-06-11 8:03 GMT+03:00 Alkis Georgopoulos <alkisg at gmail.com>:

On 10/06/2015 05:31 μμ, David Groos wrote:

Thanks Alkis for this information!

Questions:
--I could put 2 hard drives on the classroom server and install the
system on one HD and /home on another HD. Seems like that would
significantly improve performance during those times when some clients
were booting and others were logging in, but that's just an idea. Your
guess/knowledge on this?


The root file system (/) doesn't matter much as it's aggressively cached in RAM, provided of course that your server does have enough RAM.
But it's nice to put /home in an SSD, since many users need write access there in parallel. I have no benchmarks about that yet though.

 

It's also possible to use SSD as a cache layer for a normal spinning drive using dm-cache (or bcache, etc.). dm-cache has worked well for us on /home partitions. Writes are fast and most of the user data seems to sit there unused, so also most reads are fast.

 

In our case we have home partition on LVM and SSD drive is used as a cache layer. Those are then combined to cached-home device that is mounted as /home. We wrote some tools to manage the dm-cache partitions (https://github.com/opinsys/dmcache-utils), but I've understood that there's now also lvmcache.

 

 

Veli-Matti

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