mdmfs(8): Add -k skel option to populate fs from a skeleton

[kevans]

Summary

mdmfs(8) lacks the ability to populate throwaway memory filesystems from an existing directory.

This features permits an interesting setup where /var for instance lives on a device where wear-leveling is something you want to avoid as much as possible and nonetheless you don't want to lose your logs, ports metadata, etc. Here are the steps:

1. Copy /var to /var.bak;
2. Mount an mfs into /var using -k /var.bak at startup;
3. Synchronize /var to /var.bak weekly and on shutdown.

Note that this more or less mimics OpenBSD's mount_mfs(8) -P flag.

PR: 146254
Submitted by: jlh

[bcr]

OK from manpages.