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Journal Size Calculator

Work out journal size instantly with clear inputs, formula shown and shareable results.

About Journal Size Calculator

systemd-journald caps the journal at the smaller of SystemMaxUse and 10 percent of the filesystem by default, and rotates the oldest entries out when the cap is reached. Retention is therefore the cap divided by the daily write rate, not whatever MaxRetentionSec says — the size limit always wins.

Formula

Journal retention

needed = daily volume x retention days; allowed = filesystem size x SystemMaxUse share; achievable days = allowed / daily volume

Worked example

With the default values below, Journal Size Calculator returns Journal size for the retention target of 25.15 GiB. Change any input above and every figure here is recalculated in your browser.

Inputs used

Journal data written per dayminimum 0.001 MB
900 MB
Retention targetminimum 0.1 days
30 days
Filesystem size holding the journalminimum 0.001 GB
100 GB
SystemMaxUse as a share of the filesystemaccepts 0.1–100 %
10 %

Results

Journal size for the retention target
25.15 GiB
Size the configured limit allows
9.31 GiB
Retention the limit actually delivers
11.1 days
Assessment
Limit too small - raise SystemMaxUse or forward logs to a central collector

Step by step

  1. 1Values used
  2. 2Journal retention
  3. 3Journal size for the retention target= 27,000,000,000
  4. 4Size the configured limit allows= 10,000,000,000
  5. 5Retention the limit actually delivers= 11.1 days
  6. 6Assessment= Limit too small - raise SystemMaxUse or forward logs to a central collector

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

Why is my journal shorter than configured?

Because SystemMaxUse or the 10 percent default caps it by size. MaxRetentionSec only sets an upper bound on age, it does not reserve space.

Should the journal be persistent?

Yes for anything you need to investigate after a reboot. The volatile default in /run loses everything on restart, which is exactly when you want the logs.

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