Links:
Commit 0dc332bff200 URL: https://cgit.freebsd.org/src/commit/?id=0dc332bff200c940edc36c4715b629a2e1e9f9ae
Commit 9e202d036dd6 URL: https://cgit.freebsd.org/src/commit/?id=9e202d036dd6f38ce0f578aa2086ebc358315bab
Commit 5ee2c35751ef URL: https://cgit.freebsd.org/src/commit/?id=5ee2c35751ef5d131222423bf3e25073f997c337
OpenZFS Pull Request #12458 URL: https://github.com/openzfs/zfs/pull/12458

Contact: Ka Ho Ng <khng@FreeBSD.org>

Hole-punching functionality allows turning a contiguous range of bytes into a hole for a given file. File systems supporting hole-punching may deallocate the file system space from the given file. One use case for the functionality is to convert TRIM/UNMAP/DEALLOCATE requests from a virtual machine guest to hole-punching calls on the host’s side, thus allows reclamation of file system space when unneeded by the guest.

A set of APIs and KPIs are added that developers can call to invoke hole-punching on a given file, if the underlying file system exposes that functionality. For file systems not supporting hole-punching there is a fallback implementation in the kernel which does zero-filling instead. Besides the APIs and KPIs addition, the truncate(1) utility is expanded by adding a -d flag to support invoking hole-punching as well.

At the time of writing this report, OpenZFS and tmpfs are the file systems that support hole-punching.

Sponsor: The FreeBSD Foundation