[DRBD-user] Explained: Digest integrity check FAILED

Walter Haidinger walter.haidinger at gmx.at
Fri Mar 11 11:45:47 CET 2011


> > One last question for clarification, though:
> > Given the above, even online verify isn't free of false positives,
> right?
> > Then some out-of-sync blocks are to be expected. If so, a warning in the
> > manpage and guide too would probably avoid some gray hair. ;-)
> 
> Uhm. Well.
> Online verify does _not_ suffer from _this_ problem.

Ok. Problem is, the weekly verify usually sees several (about 20-30 for a 700 GiB drbd device) blocks out-of-sync.

> The pages used to read in and compare data are private to DRBD in this
> case, and we won't modify them while we are calculating checksums.
> Application IO is locked out while we are reading the data for checksums.
> 
> So if DRBD Online Verify finds out-of-sync blocks,
> they have been out of sync at that point in time.
> 
> They may be unlucky enough to see blocks where data has been modified in
> flight, and as a result local and remote disk contain differing blocks.

So, not necessarily _this_ problem but a similar one...
 
> Such differences should be very short lived, though, 
> as modification means re-dirtying, and that means
> the page will be resubmitted by upper layers "soon".
> 
> Unless these block belong to files that are unlinked before the
> respective re-dirtied page is written out again (and thus the
> re-dirtied page is simply discarded, before being re-submitted).

Hence swap is listed first in the man-page.
Databases are probably also more likely for causing this, right?

> Conclusion:
> out-of-sync blocks found by online verify, or software raid1
> "resilvering", any similar procedures, do not necessarily mean
> broken hardware or memory corruption.

The conclusion from the user perspective then is:
Verify must not find any out-of-sync blocks _only_ if everything
on top of drbd is idle (i.e. read-only or unmounted).

Lars, thanks again to clarify this issue!
IMHO you should add this to the documentation of the data integrity section.

Best regards,
Walter


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