Reducing UFS corruption from unclean shutdowns?

[Scott Long]

> On Jun 21, 2019, at 4:37 PM, Warner Losh <imp at bsdimp.com> wrote:
> 
> On Fri, Jun 21, 2019, 3:33 PM Conrad Meyer <cem at freebsd.org> wrote:
> 
>> On Fri, Jun 21, 2019 at 2:55 PM Alan Somers <asomers at freebsd.org> wrote:
>>> I would've thought that immediately following a sync(8), the
>>> filesystem would be consistent.  Why do I still see errors after a
>>> panic in files that were written before I sync()ed?
>>> -Alan
>> 
>> Hi Alan,
>> 
>> Contra the name, sync(2) (sync(8)) isn't synchronous.  It invokes
>> VFS_SYNC() with MNT_NOWAIT across all mountpoints.
>> 
> 
> Yes. Sync(2) just starts the I/O, but it may be delayed if there is a lot
> of dirty buffers. The other issue is that new buffers may be dirtied…
> 

Still, the point of SU and SU+J is that the filesystem should not be
damaged and require active repair on reboot, whether or not a
sync or fsync was done.  There’s certainly issues with disk lying
about out of order writes, POSIX sematics of unlinked files, and the
inherent design of UFS superblock updates, but the problems that
Alan reported should still be looked at, they’re not expected and
they undermine the usefulness of SU+J.

Scott