[DRBD-user] Doc update and Sync performance

Philipp Reisner philipp.reisner at linbit.com
Thu Dec 31 10:06:55 CET 2009

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

Am Dienstag, 29. Dezember 2009 23:56:17 schrieb Josh Aune:
> Hi,
> Attached is a patch for the users guide.
> Also, I'm having an issue with sync performance.  I have DRBD setup between
>  two hosts using SRP.  Under optimal conditions I'm able to achieve a 500+
>  MB/s write speed, but during the initial sync after the DRBD device is
>  created the sync proceeds at less than 100 MB/s.  Below is the config
>  file.  Am I missing anything needed to bump up the initial sync speed?
> Also, all bytes on the drives on both side are zeros, so the limitation
>  seems to be in the crc32c processing.

Out of the fact that you are using SDP, I guess that you are running on
infiniband hardware. Then you are probably not concerned about the amount
of data you move through your network.

>   syncer {
>     rate 800M;
>     csums-alg crc32c;
>     verify-alg crc32c;
>   }

This "csums-alg crc32c" statement says: Please trade of CPU-cycles for
reduced network bandwidth consumption. In your context this is probably
the reason for your 100MB/s resynchronisation performance. Simply
remove the csums-alg statement.

The man page says:

       csums-alg hash-alg
           A resync process sends all marked data blocks form the source
           to the destination node, as long as no csums-alg is given.
           When one is specified the resync process exchanges hash
           values of all marked blocks first, and sends only those data
           blocks over, that have different hash values.

           This setting is useful for DRBD setups with low bandwidth
           links. During the restart of a crashed primary node, all
           blocks covered by the activity log are marked for resync. But
           a large part of those will actually be still in sync,
           therefore using csums-alg will lower the required bandwidth
           in exchange for CPU cycles.

: Dipl-Ing Philipp Reisner
: LINBIT | Your Way to High Availability
: Tel: +43-1-8178292-50, Fax: +43-1-8178292-82
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