ZFS scrub/selfheal not really working

[Dmitry Marakasov]

* Andrew Snow (andrew at modulus.org) wrote:

> > I've recently moved my ZFS pool to 6x1TB hitachi HDDs. However,
> > those turned out to be quite crappy, and tend to grow unreadable
> > sectors.  Those sectors are really nasty, cause though they are not
> > readable, they won't be marked as bad and relocated until there's
> > write failure. And write failure actually never happens - if the sector
> > is rewritten it's pervectly readable again.
> 
> It seems like its a good idea to chuck out the whole lot, after first 
> double-checking or replacing your controller, cabling, and power supply. 

Yes, that's in plans. The box also reboots sometimes, loosing one of
HDDs from raid (until next power cycyle). I suspect power supply.

Anyway, it's a nice test for ZFS :)

>   ZFS can't help you :-)

No, actually in the current age of buggy hardware, ZFS is the only thing
that can help :)

> > So, my question is why doesn't ZFS rewrite those sectors with READ
> > errors during scrub?
> 
> Because of the transactional nature of ZFS it writes the fresh data in a 
> different part of the disk and then marks the old bad sectors as free.

Ok, then why does read errors pop up again after scrub, while they
should have been recovered?

Actually, I've forgotten to look into logs, and they say that ZFS
shrinks read block size (down to a sector size sometimes), so
corrupted sectors likely _are_ used for data, and they don't seem
to be recovered, while they should.

> > there's no parity available, will it narrow down read block size to read
> > the data and not the unused sectors with curruption?
> 
> Correct.  If no parity is available it will try its best to read as much 
> data as possible and return read errors up to the application layer on 
> sector failure.

Uh huh. That would be less worries if not the thing above.

-- 
Dmitry Marakasov   .   55B5 0596 FF1E 8D84 5F56  9510 D35A 80DD F9D2 F77D
amdmi3 at amdmi3.ru  ..:  jabber: amdmi3 at jabber.ru    http://www.amdmi3.ru