Technical data

Brocade ServerIron ADX Advanced Server Load Balancing Guide 51
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Other transparent cache switching options
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Resilient hashing for L7 and L4 TCS for maximum cache persistence
In order to the maximize ‘cache-hit ratio’ which is a critical metric for architectures utilizing cache
servers, most vendors of application delivery controllers (ADC) utilize a mechanism that provides
persistence for traffic flows. When a new cache server is added for increased capacity however or
when a cache server is rebooted or recovers from a failure event, then the traffic persistence to
cache is impacted. The network administrator then ends up either cleaning up the assembled
intelligence on traffic persistence or newly-available cache servers are not utilized until the next
available maintenance window.
Prior to software release 12.4.00a, the Brocade ServerIron ADX used a similar hashing mechanism
that required re-hashing of the hash table which resulted in the loss of persistency data with the
addition of new cache servers. Starting with software release 12.4.00a, the Brocade ServerIron
ADX is empowered with a new resilient hashing mechanism for Layer 7 TCS that allows the
seamless addition of newly-available cache servers while minimizing the impact on traffic
persistence, and therefore improves the cache-hit ratio.
The new mechanism also reduces the burden of full hash-table sync-up between two
high-availability (HA) peer Brocade ServerIron ADX systems. Although this new mechanism doesn't
completely eliminate loss of data to ensure cache persistency for new flows, it drastically reduces
the impact by restricting the effect to a smaller sub-set of traffic flows.
NOTE
With the ServerIron ADX release 12.4.00c, the resilient hashing mechanism for Layer 7 TCS is now
also available for Layer 4 TCS.
The default settings for this new resilient hashing mechanism are optimized to address common
Layer 4 and Layer 7 TCS deployments. There can however be situations that require modification of
the default settings to improve cache re-use efficiency. Network configurations where you might
want to alter the default settings include: where a deployment has a very small number of cache
servers or where the IP addressing scheme of cache servers is causing non-optimal use of cache.
In these situations, you can alter the default settings to see if efficiency is improved.
You can adjust the following resilient hashing mechanism settings.
Hash Size: This setting controls the overall size of internal hash. The default size is 4096. The
available settings are 2048, 4096, 8192 and 16384.
The following command is used to change the hash size.
ServerIronADX(config-tc-1)#resilient-hash hash-size 2048
Syntax: [no] resilient-hash hash-size hash-size
Hash Multiplier: This setting controls the granularity of hash usage against cache servers. The
default value is 60. The available settings are 20, 40, 60, 80 and 100.
The following command is used to change the hash multiplier.
ServerIronADX(config-tc-1#resilient-hash multiplier 100
Syntax: [no] resilient-hash multiplier hash-multiplier
The following command is used to toggle between resilient hash or bucket hash type of server
selection hashing mechanism as shown:
ServerIronADX(config)#server cache hash-method least-buckets
Syntax: server cache hash-method least-buckets | resilient