Confusing smartd messages

[Rodney W. Grimes]

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> On Thu, Jul 5, 2018 at 12:15 PM, Rodney W. Grimes <
> freebsd-rwg at pdx.rh.cn85.dnsmgr.net> wrote:
> 
> > > On Thu, Jul 5, 2018 at 11:03 AM, Wojciech Puchar <wojtek at puchar.net>
> > wrote:
> > >
> > > >
> > > >> Rewriting suspicious sectors is useless in this day and age.  HDDs and
> > > >> SSDs
> > > >> already do it internally and have for years.  Even healthy sectors get
> > > >>
> > > >
> > > > unreadable sectors cannot be rewritten by drive electronics as it
> > doesn't
> > > > know what to rewrite. it may possibly remap it but still report read
> > error
> > > > until some data will be written - unless giving no error and returning
> > > > meaningless data is an accepted behaviour.
> > > >
> > >
> > > But if that disk is already managed by ZFS, the pool is redundant, and
> > the
> > > bad sector is allocated by ZFS, then ZFS will immediately rewrite the
> > > unreadable sector.
> >
> > ZFS, if it gets a re error, will rewrite the unreadable sector
> > to a DIFFERENT block, not over the top of the bad spot.
> >
> 
> Are you sure?  For read errors, I think ZFS rewrites the data in-place, so
> it doesn't have to rewrite it on all other members of the same mirror/raid
> group.  For persistent write errors of course, it would have to move it to
> a different LBA as you describe.

Your right, I am not sure exactly what happens during a scrub that finds
a checksum error, or encounters a low level device I/O error.  I was wrongly
assuming that given the COW nature of the whole system that it would
never overwrite anything.

I wonder if you can send ZFS into a loop with a hard write failing sector.

> 
> >
> > > > only on write it can be done properly.
> > > >
> > > > that the HDD/SSD won't fix itself would be a checksum error.  Those are
> > > >>
> > > >
> > > > yes and this will happen if you powerdown your disk on write. or get
> > some
> > > > power spike or other source of noise that would affect electronic
> > > > components.
> > > >
> > >
> > > It happens surprisingly rarely.  Even on a sudden power loss, the drive
> > is
> > > usually able to finish its current write operation.  When you run into
> > > problems would be if the power loss were coincident with a mechanical
> > shock
> > > that knocks the head off-track, or something like that.
> >
> > I agree that "power failure" are rare causes of write errors, and an
> > idea of how often this might of happened is look at the emergency
> > retract counter, if your gettng lots of those you should try to find
> > out why and stop that.   Vibration has become a serious problem though,
> > at todays head flight hight drives are sensitive to this, you can
> > even cause a drive to do retires by yelling at it with a loud
> > voice :-)   Look at the "high fly" counter to see if your getting
> > this issue.
> >
> > > > performing full disk rewrite (so not zfs rebuilds) and THEN looking at
> > > > smart stats and THEN performing regular smartctl -t long will tell the
> > > > truth.
> > > >
> > > > which usually is "drive is fine" in my practice. really faulty drive
> > will
> > > > QUICKLY develop new problems.
> > > >
> > >
> > > Yeah, that should make the error go away.  It takes a long time, though.
> > > With a SCSI drive, you can get the exact LBAs affected with a "READ
> > > DEFECTS" command.  But there isn't a vendor-independent equivalent for
> > > SATA, unfortunately.
> >
> > My bitch exactly about ATA missing this.  Though there are vendor specific
> > commands to get it.
> >
> > --
> > Rod Grimes
> > rgrimes at freebsd.org
> >

-- 
Rod Grimes                                                 rgrimes at freebsd.org