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Session Persistence Calculator

Size the persistence table for sticky sessions: concurrent sessions, memory footprint and how long a node takes to drain.

About Session Persistence Calculator

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

Worked example

With the default values below, Session Persistence Calculator returns Concurrent Persistent Sessions of 1,620,000. Change any input above and every figure here is recalculated in your browser.

Inputs used

New sessions per secondaccepts 1–5,000,000 sessions/s
900 sessions/s
Average session durationaccepts 1–86,400 s
1,800 s
Bytes per persistence table entryaccepts 32–8,192 bytes
256 bytes
Pool membersaccepts 1–1,024 servers
8 servers

Results

Concurrent Persistent Sessions
1,620,000
Persistence Table Memory
396 MiB
Sessions Pinned per Server
202,500
Time to Drain a Node Gracefully
30.0 minutes
Sessions Re-Pinned When a Node Fails
202,500

Step by step

  1. 1Values used
  2. 2Session Persistence
  3. 3Concurrent Persistent Sessions= 1,620,000
  4. 4Persistence Table Memory= 414,720,000
  5. 5Sessions Pinned per Server= 202,500
  6. 6Time to Drain a Node Gracefully= 30.0 minutes
  7. 7Sessions Re-Pinned When a Node Fails= 202,500

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

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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