zfs:
* Enabled by use_zfs=1 in portbuild.conf
* Populate build chroots by cloning a zfs snapshot instead of maintaining
many duplicate copies. In principle this is very efficient since
everything is copy-on-write and zfs snapshot creation is almost
instantaneous. There might be additional overheads from building on zfs
though. Currently the snapshot base is hard-wired to y/${branch}@base
but should be parametrized. This also must be populated beforehand, e.g.
during machine startup
* Clean build chroots by just destroying the snapshot.
tmpfs:
* Enabled by use_tmpfs=1 and tmpfs_size in portbuild.conf
* The previous md strategy of mounting in used/, populating and then
remounting (to avoid possible races from multiple builds claiming the
same chroot) doesn't work here because tmpfs instances are destroyed at
umount. I am not entirely sure the simpler approach is free from races.
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