У меня тут вопрос возник, а можно ли им пользоваться?
В LINT (FreeBSD 4.10) и в NOTES (5.3) обнаружил следующее:
# NB: The NULL, PORTAL, UMAP and UNION filesystems are known to be
# buggy, and WILL panic your system if you attempt to do anything with
# them. They are included here as an incentive for some enterprising
# soul to sit down and fix them.
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С уважением,
Roman