Size the persistence table for sticky sessions: concurrent sessions, memory footprint and how long a node takes to drain.
A persistence record lives for as long as the session plus its timeout, so the table size is the session arrival rate multiplied by that lifetime. Draining a node gracefully means waiting for its pinned sessions to expire, which takes one full session lifetime. Persistence tables are the memory hog on an ADC and the reason maintenance windows stretch: with a 30-minute timeout you cannot take a node out cleanly in less than 30 minutes.
Session Persistence
concurrent sessions = new sessions per second × average session duration (Little's law); table memory = concurrent sessions × bytes per entry.
concurrent sessions = new sessions per second × average session duration (Little's law); table memory = concurrent sessions × bytes per entry. A persistence record lives for as long as the session plus its timeout, so the table size is the session arrival rate multiplied by that lifetime. Draining a node gracefully means waiting for its pinned sessions to expire, which takes one full session lifetime.
Persistence tables are the memory hog on an ADC and the reason maintenance windows stretch: with a 30-minute timeout you cannot take a node out cleanly in less than 30 minutes.
This calculator takes 4 inputs: New sessions per second, Average session duration, Bytes per persistence table entry, Pool members. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.