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Kernel Parameter Sizing Calculator

Work out kernel parameter sizing instantly with clear inputs, formula shown and shareable results.

About Kernel Parameter Sizing Calculator

vm.dirty_ratio caps how much page cache may hold unwritten data before writers are blocked, and on a large-memory system the default 20 percent can be many gigabytes — which takes tens of seconds to flush and produces long write stalls. Sizing it so a full flush completes in a few seconds is the usual fix on database and log-heavy servers.

Formula

Kernel sizing

fs.file-max ~ 3 x concurrent connections; dirty bytes = RAM x dirty_ratio; flush time = dirty bytes / disk write throughput

Worked example

With the default values below, Kernel Parameter Sizing Calculator returns Recommended fs.file-max of 300,000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Total RAMminimum 0.1 GB
64 GB
Concurrent TCP connectionsminimum 1
100,000
vm.dirty_ratioaccepts 1–90 %
20 %
Disk write throughputminimum 1 MB/s
500 MB/s

Results

Recommended fs.file-max
300,000
Dirty page cache the ratio permits
11.92 GiB
Time to flush a full dirty cache
25.60 s
Recommended net.core.somaxconn
5,000

Step by step

  1. 1Values used
  2. 2Kernel sizing
  3. 3Recommended fs.file-max= 300,000
  4. 4Dirty page cache the ratio permits= 12,800,000,000
  5. 5Time to flush a full dirty cache= 25.60 s
  6. 6Recommended net.core.somaxconn= 5,000

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 2 decimal places, 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 do write stalls happen?

When dirty pages reach dirty_ratio the kernel blocks the writing process until enough data is flushed. With gigabytes of dirty data and a slow disk that pause is measured in seconds.

Should dirty_bytes be used instead of dirty_ratio?

On large-memory machines yes. An absolute byte limit is independent of RAM size and gives predictable flush duration.

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