zfs i/o error, no driver error

[Martin Simmons]

>>>>> On Mon, 7 Jun 2010 05:19:54 -0700, Jeremy Chadwick said:
> 
> Which brings us to the topic of scrub interval...
> 
> This exact question was asked on the ZFS OpenSolaris list[6] in late
> 2008, and nobody there provided any concrete evidence either.  The
> closest thing to evidence is this:
> 
> "...in normal operation, ZFS only checks data as it's read back from the
> disks.  If you don't periodically scrub, errors that happen over time
> won't be caught until I next read that actual data, which might be
> inconvenient if it's a long since the initial data was written".

The question can't be answered with absolute numbers, because it depends on
other factors such as environmental effects.


> The topic of scrub intervals was also brought up a month later[7].
> Someone said:
> 
> "We did a study on re-write scrubs which showed that once per year was a
> good interval for modern, enterprise-class disks.  However, ZFS does a
> read-only scrub, so you might want to scrub more often".
> 
> The first part conflicts with what the guide recommends (I'd also like
> to see the results of the study!), while the last half of the paragraph
> makes no sense ("because it reads, do it more often!").  So if you take
> the first sentence and apply it to what the ZFS Best Practices Guide
> says, you come out with... "scrub consumer-grade disks every 6 months".

It doesn't conflict if you agree that freshly written data is more likely to
be readable that data written long ago (with some curve in between).

The re-write scrub they are talking about will write all of the data back to
the disks during the scrubbing operation, which makes it fresher.

ZFS OTOH performs read-only scrubs, i.e. it just checks that the data can be
read.  It only writes if there was a problem reading from one of the disks.

I don't know if there is any science behind that theory...

__Martin