So">?}?\, a lot of people have already answered this in various ways.
I'm going to provide a little bit of direct answer and focus to some of
those other answers (and emphasis)
On 10/12/2012 5:07 PM, Michael Stapleton wrote:
It is easy to understand that zfs srubs can be useful, But, How often do
we scrub or the equivalent of any other file system? UFS? VXFS?
NTFS? ...
ZFS has scrubs as a feature, but is it a need? I do not think so. Other
file systems accept the risk, mostly because they can not really do
anything if there were errors.
That's right. They cannot do anything. Why is that a good thing? If you
have a corruption on your filesystem because a block or even a single
bit went wrong, wouldn't you want to know? Wouldn't you want to fix it?
What if a number in an important financial document changed? Seems
unlikely, but we've discovered at least 5 instances of spontaneous disk
data corruption over the course of a couple of years. zfs corrected them
transparently. No data lost, automatic, clean, and transparent. The
more data that we make, the more that possibility of spontaneous data
corruption becomes reality.
It does no harm to do periodic scrubs, but I would not recommend doing
them often or even at all if scrubs get in the way of production.
What is the real risk of not doing scrubs?
data changing without you knowing it. Maybe this doesn't matter on an
image file (though a jpeg could end up looking nasty or destroyed, and
mpeg4 could be permanently damaged, but in a TIFF or other uncompressed
format, you'd probably never know)
Risk can not be eliminated, and we have to accept some risk.
For example, data deduplication uses digests on data to detect
duplication. Most dedup systems assume that if the digest is the same
for two pieces of data, then the data must be the same.
This assumption is not actually true. Two differing pieces of data can
have the same digest, but the chance of this happening is so low that
the risk is accepted.
but, the risk of data being flipped once you have TBs of data is way
above 0%. You can also do your own erasure coding if you like. That
would be one way to achieve the same affect outside of ZFS.
I'm only writing this because I get the feeling some people think scrubs
are a need. Maybe people associate doing scrubs with something like
doing NTFS defrags?
NTFS defrag would only help with performance. scrub helps with
integrity. Totally different things.
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