[Pacemaker] Two master colocation question

Robert Heinzmann (ml) reg at elconas.de
Mon Aug 25 18:57:45 EDT 2008

We probably mean the same thing (I hope). Practically speaking a acid 
compliant database system does the following (abstract actions) when 
commiting a transaction:

1) writing to the log
2) flush the log ((and wait for the write to finish)
3) do other things
4) before deleting the log / overwriting the log / if there is time and 
nothing else to do write to the datafile
5) remove the transaction from the log

So write order consistency means: For applications expecting local disk 
semantics (like the acid scenario described above), protocol C hehaves 
like local disk.

This means it is safe to place database log files and data files on 
different drbd devices as long as the protocol is C.

In't it ?


> Protocol C ensures write-ordering and consistency for one drbd instance
> (the local and the remote drive); not across two drbds.
> But that's neither worse nor better than two independent local disks,
> which exhibit exactly the same behaviour, and has to be handled at the
> application level.
> Regards,
>     Lars

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