[DRBD-user] Explained: Digest integrity check FAILED

Walter Haidinger walter.haidinger at gmx.at
Mon Feb 28 11:26:04 CET 2011

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.


Lars, first of all, thanks a _lot_ for this detailed explanation!
I guess this post will help many people in the future.

Could you add the warning about false positives to the Users Guide at
http://www.drbd.org/users-guide/s-integrity-check.html too?

> Quoting the DRBD User's Guide:
>   Notes on data integrity
>
>   This may happen for swap, or for certain append while global sync, or
>   truncate/rewrite workloads, and not necessarily poses a problem for the
>   integrity of the data. Usually when the initiator of the data transfer
>   does this, it already knows that that data block will not be part of an
>   on disk data structure, or will be resubmitted with correct data soon
>   enough.

This is new in drbd.conf(5) of 8.3.10. Unfortunately I was only aware
of 8.3.9 which just mentions swap and ReiserFS. Having neither of those
two, I thought I was "free" of false positives...

> If you want to have DRBD do "end-to-end" data checksums, even if the
> data buffers may be modified while being in flight, and still want it to
> be efficient, sponsor feature development.

Fair enough! ;-)
 
> The Problem:
>   http://lwn.net/Articles/429305/
>   http://thread.gmane.org/gmane.linux.kernel/1103571
>   http://thread.gmane.org/gmane.linux.scsi/59259
> 
> And many many more older threads on various ML,
> some of them misleading, some of them mixing
> this issue of in-flight modifications
> with actual (hardware caused) data corruption.

This and the (outdated) notes of drbd.conf(5) probably got me
on the "wrong" track. It also explains why I was unable to reproduce 
any data corruption.

> Possible Solutions:
[...]

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. ;-)

Thanks again and best regards,
Walter

-- 
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