Brocade Virtual ADX Server Load Balancing Guide (Supporting ADX v03.1.00) - Manual (Page 105)

Brocade Virtual ADX Server Load Balancing Guide 89
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Hash-based SLB with server persistence
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Configuring the reassign threshold and duration
There are two configurable global parameters related to the reassign mechanism:
• Reassign threshold — When the number of empty hash entries (buckets) in the persistent hash
table falls to or below this threshold (less than or equal to), the Brocade Virtual ADX reassigns
some of the persistent hash entries on introduction of a new real server. By default, the
Brocade Virtual ADX reassigns persistent hash entries to the new real server only if there are
no empty persistent hash entries (for example, the default persist hash reassign threshold is 0
percent).
To specify the threshold, enter a command such as the following.
Virtual ADX(config)#server persist-hash-threshold 5
Syntax: [no] server persist-hash-threshold percent-value
The percent-value variable can be any value from 0 through 99.
With the reassign mechanism, if multiple servers come up simultaneously and need
reassignment because the number of empty hash table entries is below the reassign
threshold, then the Brocade Virtual ADX will clear the persistent hashing table.
• Reassign duration — If the number of empty persistent hash entries is below the reassign
threshold, the Brocade Virtual ADX reassigns some of the persistent hash entries over a period
of time to the new real server. This duration of time is known as the persist hash reassign
duration.
To specify the reassign duration, enter a command such as the following.
Virtual ADX(config)#server persist-hash-reassign-duration 5
This global command is applied to all configured VIP ports that are persist-hash enabled. The
Brocade Virtual ADX will complete the reassignment within 2 minutes by default.
Syntax: [no] server persist-hash-reassign-duration value
The value variable can be a time duration from 2 to 30 minutes.
Reassignment sequence and example
The sequence is performed as follows.
1. When a new server is introduced, the Brocade Virtual ADX calculates how many of the hash
table entries in the persistent hash table are empty. If the number is greater than the
persist-hash-reassign-threshold, the Brocade Virtual ADX does no reassignment.
If the number of empty hash table entries is less than or equal to the persist hash reassign
threshold, the Brocade Virtual ADX proceeds with the reassignment. The system first
calculates the total number of active real servers, which includes the new real server.
2. The system calculates the entries per server as follows.
X = entries per server = total assigned hash table entries/number of active real servers
3. The Brocade Virtual ADX attempts to reassign X persistent hash entries to the new real server
over the duration specified by the persist-hash-reassign-duration.