[Server-devel] Server-devel Digest, Vol 10, Issue 8

[Tony Pearson]

Barry,
Here are my thoughts on redundancy.  The basic heuristics is as follows:

(a) Determine the minimum setup and identify all components that can fail 
individually.
(b) For each failure, what would be the impact to the entire system.
(c) Determine an N+1 or N+2 configuration that might address the concern.

Let's assume that:
The LS (Library Server) is not located at the school, but somewhere else 
in the country, or hosted by the ISP itself.
There is a single ISP provider, who provides a single line to the school 
that carries the internet signal.
The ISP provides a single RJ45 terminal cable, which can be connected to a 
Hub or XS server
A single four-port hub/Wifi router, which allows laptop use, network 
printer, or other servers.
The School has a single XS server, two USB ports, two Active Antenna, and 
that each antenna handles maximum 100 XO laptops
The WiFi router is WEP-protected so that only teachers/principals/guests 
have access, all students on mesh-only
There are 200 laptops, at least one per student or teacher.

LS----(ISP)---Hub----XS----AA1- - - - - XO1, XO2, ... XO100
                       ----AA2- - - - - XO101, XO102, ... XO200

Failure scenarios:

LS fails -- students have access to local activities, XS moodle lessons, 
and internet, and whatever LS content cached on XS server
        Is that a problem?  If there is a central LS for all of Nepal, 
they should consider a secondary LS server.
        Resolution:  Perhaps have some LS content permanently on a local 
server, either on XS or other, in event central LS is down

ISP fails -- students have access to local activities, XS moodle lessons, 
and whatever LS/Internet cached content on XS server
        What is ISP providing for "Service Level Agreement".  Can they 
resolve this in a single day or two?
        Resolution: class might continue without internet access.  Some 
lessons might be impacted that involve LS or Internet access.

Hub fails --  students lose access to LS and internet.  Teachers lose 
access to WiFi.
        Resolution: XS could be connected directly to ISP until new Hub 
replacement made available.  Students continue as before.

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.
        The XS failure could be either the disk drive itself fails, or 
something else on the system that prevents it from running.
        Resolution: it would seem that best option is multiple XS servers, 
and perhaps mirrored disk data between the two systems.

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
        many XO laptops an active antenna can support?
        If AA1 was for second graders, and AA2 was for sixth graders, then 
perhaps only one grade impacted.
        Resolution:  Having an AA3 would mean that any one antenna 
failure, the remaining two antenna can handle the workload

XO (teacher) fails -- An individual teacher is impacted.   For N teachers, 
you should consider N+1 XO laptops, with one or more spare
        to handle this situation.  The teacher XO would be enabled for 
WiFi-WEP key and have whatever extra software was needed on them.
        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.

XO (student) fails -- An individual student forgets his XO at home, breaks 
it, or whatever.  Too bad.  Student looks over shoulder of
        a fellow student.  Alternatively, have a few XO student laptops 
that can be swapped out with the broken one while the broken
        one is getting repaired.  Student would lose any work unless it 
was backed up to XS server.

Here is an alternative with some redundancy built in:

LS1----(ISP)----Hub------------------------------------------Hub
LS2             XS----AA1, AA2, AA3

In this case, we have two Library Servers in the central location, and the 
ISP or the LS-folks
handle this so that they are properly available if one or the other is 
down.

Alternatives for disk failure can include a LiveCD+USB stick.  In this 
case, if the disk fails, you
boot from a LiveCD, and the USB stick has all the modified values (conf 
files, IP settings, etc).
Depending on the size of the USB stick, could contain critical backups of 
Moodle lesson plans, etc.
There are also ways to have a "Boot from USB stick" that can then have 
either all the modifications
needed, or a second USB with the modified values.

Fedora 7 uses Linux LVM, and I suspect this level of LVM supports disk 
mirroring, which makes updates
to two disks at the same time.  In the event a single disk fails, the 
other disk would be used.  That
needs to be investigated.  In this case, there would be two disks inside 
one server, containing identical
information.

Three Active Antenna would handle 300 laptops, so losing one can still 
handle the 200 XO laptops expected.
A second hub provides wider WiFi access, more ports for 
peripherals/printers, etc.  In the event a hub
fails, the other one can be connected in its place.

For this to work, you need an XS server with at least 3 USB ports.

Another alternative:
LS1----(ISP)----Hub------------------------------------------Hub
                XS1----AA1, AA2                              XS2----AA3, 
AA4
                LS2-Local

In this example, LS2 is a local version or subset of LS1, in the event the 
internet or LS1 is down,
the LS2 can be used instead.

There can be two XS servers.  XS1 for second graders, XS2 for sixth 
graders.  In the event either
one is down, all students can use the remaining XS server.  Each AA can 
handle 100 XO laptops, so
assuming 100 second graders and 100 sixth graders, then this setup can 
handle loss of any two active
antenna and still be able to handle all students, and provide room for 
growth.  XS1 and XS2 would
send backups to each others databases to each other as needed, and if 
needed, an XS could handle
the databases of both sets of students, and possibly have separate 
Moodle's on single Apache instance.

XS1 could backup to XS2 and vice versa.  This can be scheduled with CRON 
and SCP. 
Alternatively, LS2 could double as the backup server, with XS1 and XS2 
sending backups to LS2.

 
 

Tony Pearson
(IBM)
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