Use WAL mode for SQLite cache databases#150
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grahamc merged 3 commits intodetsys-mainfrom Jul 16, 2025
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Instead of error (ignored): error: SQLite database '...' is busy we now get error (ignored): SQLite database '...' is busy
With "truncate" mode, if we try to write to the database while another process has an active write transaction, we'll block until the other transaction finishes. This is a problem for the evaluation cache in particular, since it uses long-running transactions. WAL mode does not have this issue: it just returns "busy" right away, so Nix will print error (ignored): SQLite database '/home/eelco/.cache/nix/eval-cache-v5/...' is busy and stop trying to write to the evaluation cache. (This was the intended/original behaviour, see AttrDb::doSQLite().)
grahamc
approved these changes
Jul 16, 2025
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Motivation
With "truncate" mode, if we try to write to the database while another process has an active write transaction, we'll block until the other transaction finishes. This is a problem for the evaluation cache in particular, since it uses long-running transactions.
WAL mode does not have this issue: it just returns "busy" right away, so Nix will print
and stop trying to write to the evaluation cache. (This was the intended/original behaviour, see
AttrDb::doSQLite().)A quick benchmark (running
nix search github:NixOS/nixpkgs/bf8462aeba50cc753971480f613fbae0747cffc0?narHash=sha256-bPyv7hsbtuxyL6LLKtOYL6QsmPeFWP839BZQMd3RoUg%3D20 times) did not show a significant performance difference:Context