Session Persistence Calculator
Size the persistence table for sticky sessions: concurrent sessions, memory footprint and how long a node takes to drain.
Inputs
Concurrent Persistent Sessions
1,620,000
Persistence Table Memory
396 MiB
Sessions Pinned per Server
202,500
Time to Drain a Node Gracefully
30.0minutes
Sessions Re-Pinned When a Node Fails
202,500
Step by step
Values used
New sessions per second = 900 sessions/s; Average session duration = 1,800 s; Bytes per persistence table entry = 256 bytes; Pool members = 8 servers
Session Persistence
concurrent sessions = new sessions per second × average session duration (Little's law); table memory = concurrent sessions × bytes per entry.
Concurrent Persistent Sessions
= 1,620,000
Persistence Table Memory
= 414,720,000
Sessions Pinned per Server
= 202,500
Time to Drain a Node Gracefully
= 30.0 minutes
Sessions Re-Pinned When a Node Fails
= 202,500
How it works
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.
Formula
Session Persistence
concurrent sessions = new sessions per second × average session duration (Little's law); table memory = concurrent sessions × bytes per entry.
- sessionDuration
- Includes the persistence timeout, not just active use
- entryBytes
- Source address or cookie value plus the pool member reference
Frequently Asked Questions
How is Session Persistence calculated?
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.
Why does Session Persistence matter?
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.
What values do I need to enter?
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.
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