|SUBUID(5)||File Formats and Conversions||SUBUID(5)|
This file specifies the user IDs that ordinary users can use, with the newuidmap command, to configure uid mapping in a user namespace.
Multiple ranges may be specified per user.
When large number of entries (10000-100000 or more) are defined in /etc/subuid, parsing performance penalty will become noticeable. In this case it is recommended to use UIDs instead of login names. Benchmarks have shown speed-ups up to 20x.