Technical data

56 ServerIron ADX Advanced Server Load Balancing Guide

53-1002435-01

Other TCS options

Increasing the TCS hash bucket count

The ServerIron ADX supports 256 hash buckets by default. If you do not assign weights to the

cache servers, the software divides the hash buckets evenly among the cache servers. With the

default hash bucket count of 256; there is a limitation of traffic distribution. In a setup with large

number of cache servers, if one of the cache servers fails, then their remaining cache servers may

get hit by traffic spike due to limited load balancing by hash buckets.

Consider an example where there are 64 cache servers within one cache group(CS1-CS64). Since

there are 256 buckets; 256 / 64 = 4, each server is assigned with 4 buckets. If one cache server,

(CS1), goes down, the 4 buckets assigned to CS1 are re-assigned to cache servers “CS2-CS5”.

Consequently, “CS2-CS5” have 5 buckets each while CS6 through CS64 still have 4 buckets. This

means that the original traffic handled by the cache server going down isn’t distributed evenly

among the rest of the cache servers. The traffic on CS2-CS5 increases by ( 5 - 4 ) / 4 = 25%.

You can increase the TCS hash bucket count to a higher number to ensure a more reasonable

distribution of excess traffic among remaining cache servers when a cache server goes down.

Using the previous example where there are 64 cache servers(CS1-CS64) you can upgrade the TCS

hash bucket count to 8192. Since 8192 / 64 = 128, each server is now assigned with 128

buckets. If one cache server (CS1) goes down, then the 128 buckets assigned to CS1 are

re-assigned to the other 63 servers. In this situation, CS2 and CS3 get 3 additional buckets each

while CS4 through CS64 get 2 buckets each (2 * 3 + 61 * 2 = 128). The result is that the original

traffic handled by the cache server that went down is now distributed evenly among the remaining

functional cache servers. The traffic on CS2-CS3 increases by ( 131 - 128 ) / 128 = 2.34%, and on

CS4-CS64 ( 130 - 128) / 128 = 1.56%.

255.255.255.0 0.0.0.255 125.24.32.12 149.165.16.233 C1

125.24.32.12 189.12.122.233 C1

125.24.32.12 189.12.122.200 C2

ffff:ffff:ffff:ffff:: :: 2001:db8:0102:0303:6::1 Any C1

2001:db8:0102:0303:7::1 Any C1

2001:db8:0102:0304:8::1 Any C2

2001:db8:0102:0305:9::1 Any C3

ffff:ffff:ffff:ffff:ffff:: :: 2001:db8:0102:0303:6::1 Any C1

2001:db8:0102:0303:7::1 Any C2

2001:db8:0102:0304:8::1 Any C3

ffff:ffff:ffff:ffff:: ::ffff 2001:db8:0102:0303:6::1 2001:db8:0102:1 C1

2001:db8:0102:0303:7::1 2001:db8:0304:1 C1

2001:db8:0102:0303:8::1 2001:db8:0506:2 C2

TABLE 4 Example TCS hash masks (IPv4+IPv6) (Continued)

Destination mask Source mask Destination IP address Source IP address Cache server