[DRBD-user] Strange replication using DRBD 8.3.2

loopx laurent.henssen at gmail.com
Thu Apr 15 17:19:13 CEST 2010

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

Stefan Seifert-3 wrote:
> On Wednesday 14 April 2010 13:46:38 loopx wrote:
>> http://www.drbd.org/docs/introduction/ :
>>     Protocol C. Synchronous replication protocol. Local write operations
>> on
>> the primary node are considered completed only after both the local and
>> the
>> remote disk write have been confirmed. As a result, loss of a single node
>>  is guaranteed not to lead to any data loss. Data loss is, of course,
>>  inevitable even with this replication protocol if both nodes (or their
>>  storage subsystems) are irreversibly destroyed at the same time. By far,
>>  the most commonly used replication protocol in DRBD setups is protocol
>> C.
>> -------------
>> But, when using 8.3.2 (on both side), and copy a 10Gb (9.6 in fact) file
>> from LOCAL (/) to DRBD0 (mounted under /mnt/), I see that the local copy
>> is
>> finished, but the network transfert is not yet finished and didn't
>> receive
>> the complete file (see the amount of date received by using "iftop") ...
> The solution is very simple: while drbd waits for the remote side to
> complete 
> writing before signaling a write completed, cp itself does not wait! cp
> just 
> issues the write request, but does not wait for it to complete. So the
> missing 
> data is still in your local write cache waiting to be processed by drbd
> and 
> your disks.
> You can see the same with pure local writes, although it's harder to see 
> because a local disk is writing quicker: cp is finished while your disk is 
> still working. To be sure that the write is completed, issue a sync after
> your 
> cp or use something a bit more sophisticated for your tests like dd as has 
> been explained quite a few times on this list.
> Regards,
> Stefan Seifert
> _______________________________________________
> drbd-user mailing list
> drbd-user at lists.linbit.com
> http://lists.linbit.com/mailman/listinfo/drbd-user

Thanks you very much for you answer and sorry about the already know
"behavior". I successfully completed some test with "cp + sync" and "dd +
fsync" and all is now fine when I broke the network after the command return
to the shell ;-).

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