Jens Frank wrote:
Clustering two web servers will increase the web servers availability to 99%. But now the system has three components: Load balancer (90%), Web servers (99%), database (90%). That's a total availability of 80.19%.
Jens, have you ever seen a real load balancer that had as little as 90% availability, or are you just dreaming up some numbers that will prove your prejudice?
Here are my recent observations for www.wikipedia.org/wiki/Sweden (slowness means response time was 5-60 seconds, downtime means no answer at all within 60 seconds, this page is circa 50 kbytes):
Week Beginning Downtime Slowness Avg access time -------- ----------- -------- -------- --------------- 2003-w44 27 Oct 2003 1 % 54 % 8.51 seconds 2003-w43 20 Oct 2003 1 % 39 % 6.75 2003-w42 13 Oct 2003 2 % 11 % 3.66 2003-w41 6 Oct 2003 2 % 22 % 3.42 2003-w40 29 Sep 2003 6 % 29 % 4.84 2003-w39 22 Sep 2003 5 % 26 % 4.06 2003-w38 15 Sep 2003 1 % 25 % 3.66 2003-w37 8 Sep 2003 5 % 36 % 4.90 2003-w36 1 Sep 2003 2 % 10 % 2.26 2003-w35 25 Aug 2003 0 % 1 % 1.42 2003-w34 18 Aug 2003 0 % 2 % 1.48 2003-w33 11 Aug 2003 0 % 12 % 2.41 2003-w32 4 Aug 2003 0 % 6 % 2.06 2003-w31 28 Jul 2003 1 % 2 % 1.52 2003-w30 21 Jul 2001 26 % 1 % 1.90
I think these numbers indicate that slowness (performance) is the problem, not low availability. The server is capable of delivering a response in 1.5 seconds, but in the last week prefers to add seven seconds of dead weight (dead wait, uhuh) to this.
If I could decide, we should build a time machine that brings us back to August. I guess this is vacation month with less traffic. Or is there any other explanation?