Confusing smartd messages

[Alan Somers]

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.


>
> 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.


>
> 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.

-Alan