<br><font size=2 face="sans-serif">Barry,</font>
<br><font size=2 face="sans-serif">Here are my thoughts on redundancy.
The basic heuristics is as follows:</font>
<br>
<br><font size=2 face="sans-serif">(a) Determine the minimum setup and
identify all components that can fail individually.</font>
<br><font size=2 face="sans-serif">(b) For each failure, what would be
the impact to the entire system.</font>
<br><font size=2 face="sans-serif">(c) Determine an N+1 or N+2 configuration
that might address the concern.</font>
<br>
<br><font size=2 face="sans-serif">Let's assume that:</font>
<br><font size=2 face="sans-serif">The LS (Library Server) is not located
at the school, but somewhere else in the country, or hosted by the ISP
itself.</font>
<br><font size=2 face="sans-serif">There is a single ISP provider, who
provides a single line to the school that carries the internet signal.</font>
<br><font size=2 face="sans-serif">The ISP provides a single RJ45 terminal
cable, which can be connected to a Hub or XS server</font>
<br><font size=2 face="sans-serif">A single four-port hub/Wifi router,
which allows laptop use, network printer, or other servers.</font>
<br><font size=2 face="sans-serif">The School has a single XS server, two
USB ports, two Active Antenna, and that each antenna handles maximum 100
XO laptops</font>
<br><font size=2 face="sans-serif">The WiFi router is WEP-protected so
that only teachers/principals/guests have access, all students on mesh-only</font>
<br><font size=2 face="sans-serif">There are 200 laptops, at least one
per student or teacher.</font>
<br>
<br><font size=2 face="Courier New">LS----(ISP)---Hub----XS----AA1- - -
- - XO1, XO2, ... XO100</font>
<br><font size=2 face="Courier New">
----AA2- - - - - XO101,
XO102, ... XO200</font><font size=2 face="sans-serif"><br>
</font>
<br><font size=2 face="sans-serif">Failure scenarios:</font>
<br>
<br><font size=2 face="sans-serif">LS fails -- students have access to
local activities, XS moodle lessons, and internet, and whatever LS content
cached on XS server</font>
<br><font size=2 face="sans-serif"> Is
that a problem? If there is a central LS for all of Nepal, they should
consider a secondary LS server.</font>
<br><font size=2 face="sans-serif"> Resolution:
Perhaps have some LS content permanently on a local server, either
on XS or other, in event central LS is down</font>
<br>
<br><font size=2 face="sans-serif">ISP fails -- students have access to
local activities, XS moodle lessons, and whatever LS/Internet cached content
on XS server</font>
<br><font size=2 face="sans-serif"> What
is ISP providing for "Service Level Agreement". Can they
resolve this in a single day or two?</font>
<br><font size=2 face="sans-serif"> Resolution:
class might continue without internet access. Some lessons might
be impacted that involve LS or Internet access.</font>
<br>
<br><font size=2 face="sans-serif">Hub fails -- students lose access
to LS and internet. Teachers lose access to WiFi.</font>
<br><font size=2 face="sans-serif"> Resolution:
XS could be connected directly to ISP until new Hub replacement made available.
Students continue as before.</font>
<br>
<br><font size=2 face="sans-serif">XS fails -- students lose access to
LS, Internet and XS moodle. Can they mesh with each other?
They can continue using activities on their XO.</font>
<br><font size=2 face="sans-serif"> The
XS failure could be either the disk drive itself fails, or something else
on the system that prevents it from running.</font>
<br><font size=2 face="sans-serif"> Resolution:
it would seem that best option is multiple XS servers, and perhaps mirrored
disk data between the two systems.</font>
<br>
<br><font size=2 face="sans-serif">AA1 fails -- If one Active Antenna fails,
the other Active Antenna will not be able to handle the total 200 XO laptops.
Do we know how</font>
<br><font size=2 face="sans-serif"> many
XO laptops an active antenna can support?</font>
<br><font size=2 face="sans-serif"> If
AA1 was for second graders, and AA2 was for sixth graders, then perhaps
only one grade impacted.</font>
<br><font size=2 face="sans-serif"> Resolution:
Having an AA3 would mean that any one antenna failure, the remaining
two antenna can handle the workload</font>
<br>
<br><font size=2 face="sans-serif">XO (teacher) fails -- An individual
teacher is impacted. For N teachers, you should consider N+1 XO
laptops, with one or more spare</font>
<br><font size=2 face="sans-serif"> to
handle this situation. The teacher XO would be enabled for WiFi-WEP
key and have whatever extra software was needed on them.</font>
<br><font size=2 face="sans-serif"> In
lieu of an XO, the teacher could have a full PC running QEMU emulating
the XO image, in the event it takes long to repair the original XO.</font>
<br>
<br><font size=2 face="sans-serif">XO (student) fails -- An individual
student forgets his XO at home, breaks it, or whatever. Too bad.
Student looks over shoulder of</font>
<br><font size=2 face="sans-serif"> a
fellow student. Alternatively, have a few XO student laptops that
can be swapped out with the broken one while the broken</font>
<br><font size=2 face="sans-serif"> one
is getting repaired. Student would lose any work unless it was backed
up to XS server.</font>
<br>
<br><font size=2 face="sans-serif">Here is an alternative with some redundancy
built in:</font>
<br>
<br><font size=2 face="Courier New">LS1----(ISP)----Hub------------------------------------------Hub</font>
<br><font size=2 face="Courier New">LS2
XS----AA1, AA2, AA3</font>
<br>
<br><font size=2 face="Courier New">In this case, we have two Library Servers
in the central location, and the ISP or the LS-folks</font>
<br><font size=2 face="Courier New">handle this so that they are properly
available if one or the other is down.</font>
<br>
<br><font size=2 face="Courier New">Alternatives for disk failure can include
a LiveCD+USB stick. In this case, if the disk fails, you</font>
<br><font size=2 face="Courier New">boot from a LiveCD, and the USB stick
has all the modified values (conf files, IP settings, etc).</font>
<br><font size=2 face="Courier New">Depending on the size of the USB stick,
could contain critical backups of Moodle lesson plans, etc.</font>
<br><font size=2 face="Courier New">There are also ways to have a "Boot
from USB stick" that can then have either all the modifications</font>
<br><font size=2 face="Courier New">needed, or a second USB with the modified
values.</font>
<br>
<br><font size=2 face="Courier New">Fedora 7 uses Linux LVM, and I suspect
this level of LVM supports disk mirroring, which makes updates</font>
<br><font size=2 face="Courier New">to two disks at the same time. In
the event a single disk fails, the other disk would be used. That</font>
<br><font size=2 face="Courier New">needs to be investigated. In
this case, there would be two disks inside one server, containing identical</font>
<br><font size=2 face="Courier New">information.</font>
<br>
<br><font size=2 face="Courier New">Three Active Antenna would handle 300
laptops, so losing one can still handle the 200 XO laptops expected.</font>
<br><font size=2 face="Courier New">A second hub provides wider WiFi access,
more ports for peripherals/printers, etc. In the event a hub</font>
<br><font size=2 face="Courier New">fails, the other one can be connected
in its place.</font>
<br>
<br><font size=2 face="Courier New">For this to work, you need an XS server
with at least 3 USB ports.</font>
<br>
<br><font size=2 face="Courier New">Another alternative:</font>
<br><font size=2 face="Courier New">LS1----(ISP)----Hub------------------------------------------Hub</font>
<br><font size=2 face="Courier New">
XS1----AA1, AA2
XS2----AA3,
AA4</font>
<br><font size=2 face="Courier New">
LS2-Local</font>
<br>
<br><font size=2 face="Courier New">In this example, LS2 is a local version
or subset of LS1, in the event the internet or LS1 is down,</font>
<br><font size=2 face="Courier New">the LS2 can be used instead.</font>
<br>
<br><font size=2 face="Courier New">There can be two XS servers. XS1
for second graders, XS2 for sixth graders. In the event either</font>
<br><font size=2 face="Courier New">one is down, all students can use the
remaining XS server. Each AA can handle 100 XO laptops, so</font>
<br><font size=2 face="Courier New">assuming 100 second graders and 100
sixth graders, then this setup can handle loss of any two active</font>
<br><font size=2 face="Courier New">antenna and still be able to handle
all students, and provide room for growth. XS1 and XS2 would</font>
<br><font size=2 face="Courier New">send backups to each others databases
to each other as needed, and if needed, an XS could handle</font>
<br><font size=2 face="Courier New">the databases of both sets of students,
and possibly have separate Moodle's on single Apache instance.</font>
<br>
<br><font size=2 face="Courier New">XS1 could backup to XS2 and vice versa.
This can be scheduled with CRON and SCP. </font>
<br><font size=2 face="Courier New">Alternatively, LS2 could double as
the backup server, with XS1 and XS2 sending backups to LS2.</font>
<br>
<br><font size=2 face="Courier New">
</font>
<br><font size=2 face="Courier New">
</font>
<br>
<br><font size=2 face="sans-serif">Tony Pearson</font>
<br><font size=2 face="sans-serif">(IBM)</font>