Hello,
If there is one, what is the optimal number of AFS database servers
to have in a growing cell?
What are the factors which determine this?
(I am assuming pure database servers. Eg they don't also fileserve.)
+-number of database servers (N) -------------------------------------[1]
|
| +-quorum (eg need Q out of N servers to elect sync site)----------[2]
| |
| | +- 1 server failed, chance of client connecting to server OK--[3]
| | |
| | | + "diminishing return"----------------------[4]
| | | |
1 1 0/1 -> 0%
2
3 2 2/3 -> 66% 66
4
5 3 4/5 -> 80% 14
6
7 4 6/7 -> 85% 5
8
9 5 8/9 -> 88% 3
10
11 6 10/11 -> 90% 2
12
13 7 12/13 -> 92% 2
14
15 8 14/15 -> 93% 1
Other factors:
a) It seems that it is best to have an odd number of db servers so
that in the event of the sync site server failing the remaining
db servers can elect a new sync site.
b) Also, while it might seem to be good to have 11 db servers
(so that clients have a 90% chance of connecting to a working
server) what is the increased probability of a machine failing
having 11 instead of (say) 5?
c) The more db servers you have, the better the distribution
of processing load.
d) There is a "diminishing return" [4] as the number of servers grows
Eg, growing from 1 to 3 servers yields an increase of 66% [3]
while growing from 9 to 11 servers yields an increase of 2% [3]
e) By having "pure" db servers, better performance and reliability
is obtained (eg on db servers, switch off all surplus daemons
and servces).
Ignoring the obvious question of cost of machines, does anyone
have a way to compute the optimal number?
It seems to me that 3 is a good number but 5 might be best.
--
paul http://acm.org/~mpb
"Young man, in mathematics you don't understand things,
you just get used to them." -- John van Nuemann
"Without a quorum, we have a quandary." --unknown
