On Mon, 28 Sep 2009, Richard Elling wrote:
In other words, I am concerned that people replace good
data protection
practices with scrubs and expecting scrub to deliver better data
protection
(it won't).
Many people here would profoundly disagree with the above. There is
no substitute for good backups, but a periodic scrub helps validate
that a later resilver would succeed. A perioic scrub also helps find
system problems early when they are less likely to crater your
business. It is much better to find an issue during a scrub rather
than during resilver of a mirror or raidz.
Scrubs are also useful for detecting broken hardware. However,
normal activity will also detect broken hardware, so it is better to
think of scrubs as finding degradation of old data rather than being
a hardware checking service.
Do you have a scientific reference for this notion that "old data" is
more likely to be corrupt than "new data" or is it just a gut-feeling?
This hypothesis does not sound very supportable to me. Magnetic
hysteresis lasts quite a lot longer than the recommended service life
for a hard drive. Studio audio tapes from the '60s are still being
used to produce modern "remasters" of old audio recordings which sound
better than they ever did before (other than the master tape). Some
forms of magnetic hysteresis are known to last millions of years.
Media failure is more often than not mechanical or chemical and not
related to loss of magnetic hysteresis. Head failures may be
construed to be media failures.
See http://en.wikipedia.org/wiki/Ferromagnetic for information on
ferromagnetic materials.
It would be most useful if zfs incorporated a slow-scan scrub which
validates data at a low rate of speed which does not hinder active
I/O. Of course this is not a "green" energy efficient solution.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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