[OpenIndiana-discuss] Zfs stability "Scrubs"

Doug Hughes doug at will.to
Sat Oct 13 02:36:13 UTC 2012

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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