Re: Using ZFS uberblock time to initialise system clock

Van: Peter Jeremy <[email protected]>
Datum: 9 mei 2026 03:00
Aan: [email protected]
Onderwerp: Using ZFS uberblock time to initialise system clock

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> 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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Hi Peter,
I hope you will find the answer.
In the meantime I can advertise my port that “resolved” this issue for me.
https://www.freshports.org/sysutils/fakertc/

Regards,
Ronald