Brocade Virtual ADX Server Load Balancing Guide (Supporting ADX v03.1.00) - Manual (Page 106)

90 Brocade Virtual ADX Server Load Balancing Guide
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Hash-based SLB with server persistence
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The Brocade Virtual ADX will stop reassigning persistent hash entries to the new real server when
either of the following occurs:
• The system has finished reassigning X persistent hash entries to the new real server (occurs in
the amount of time specified by the persist-hash-reassign-duration).
• The number of persistent hash entries assigned to the new real server is equal to the lowest
number of persistent hash entries assigned to any of the existing real servers, whichever
happens earlier.
Consider the following reassignment example. Figure 16 shows the hash table before
reassignment.
FIGURE 16 Hash table before reassignment
Persistent hash entries have been assigned as follows. Entries 47 to 54 have been assigned to real
server rs1. Entries 55 and 56 have been assigned to rs2. All other entries are empty (no real server
has been assigned to them).
In this example, the administrator configures a reassign-threshold of 99 percent. That is, whenever
the number of empty hash entries falls below 99 percent, the Brocade Virtual ADX will reassign the
persistent hash table entries whenever a new real server comes up. The reassign-duration is the
default value (2 minutes).
Next, the administrator binds port HTTP of a new real server rs3 to port HTTP of virtual server vs1.
When real server rs3 comes up, the Brocade Virtual ADX calculates the number of active real
server ports. In this example, the number is 3 (rs1, rs2 and rs3). The system then calculates the
number of empty hash table entries. In this example, the number is 246. Because less than 99
percent of the hash table entries are empty, the Brocade Virtual ADX now attempts to reassign
some of the persistent hash entries to the new real server rs3.
The Brocade Virtual ADX then calculates entries per server X as follows.
X = total assigned hash table entries/number of active real servers = 10/3 = 3
virtual server vs1 port http
Persistent Hash table
Hash 0 none
Hash 1 none
..............
Hash 47 rs1
Hash 49 rs1
Hash 55 rs2
Hash 53 rs1
Hash 54 rs1
Hash 56 rs2
Hash 48 rs1
Hash 50 rs1
Hash 51 rs1
Hash 52 rs1
Hash 255 none
..............