Using ZFS uberblock time to initialise system clock

Peter Jeremy <[email protected]> Sat, 9 May 2026 10:59:42 +1000

Newsgroups	gmane.os.freebsd.devel.file-systems
Message-ID	<[email protected]>

FreeBSD has provision to initialise the system TOD clock from a filesystem
timestamp if no hardware RTC provides a valid time (see inittodr() called
from vfs_mountroot() or nfs_mountroot()).  This has support for UFS and NFS
root filesystems but not ZFS.

ZFS has a suitable timestamp in its uberblock (ub_timestamp) so it should
just be a matter of plumbing that timestamp through.  Digging into the code,
the most obvious problem is that ZFS doesn't initialise mnt_time in struct
mount when the filesystem is mounted.  So I tried patching zfs_domount() to
initialise mnt_time.  That solved the plumbing problem but revealed that, by
the time zfs_domount() is called on the root filesystem, ub_timestamp has
been "updated" to the (uninitialised) system clock.

Before I spend too much more time digging into this, has anyone else dug into
where the uberblock is updated during the root mount?

-- 
Peter Jeremy

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