[DRBD-user] how long does drbd device do sync

Lars Ellenberg lars.ellenberg at linbit.com
Thu May 10 13:50:35 CEST 2007

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

On Thu, May 10, 2007 at 10:44:58AM +0100, Mark Watts wrote:
> > Hi all:
> >
> >       I have no idea about how long the primary sync its data to secondary?
> >
> > Or once the data is changed on primary physical device, it is send to
> > secondary immediately?
> Data replication is basically real-time, within the constraints of network 
> bandwidth and raw I/O performance.
> In the "resource" section of /etc/drbd.conf, you define the transfer protocol 
> to use for that resource (/dev/drbd0 for example).
> It can be one of the following three:
>    C: write IO is reported as completed, if we know it has
>       reached _both_ local and remote DISK.
>       * for critical transactional data.
       * for most cases.

>    B: write IO is reported as completed, if it has reached
>       local DISK and remote buffer cache.
       * for cases were you can live with possible data loss
         of the last few milliseconds in case of failover

we should improve our documented recommendations :)

>    A: write IO is reported as completed, if it has reached
>       local DISK and local tcp send buffer. (see also sndbuf-size)
>       * for high latency networks
> If you choose C, then write a file to the DRBD'd filesystem,
> the write won't 

and then fsync! otherwise the write goes
just into the local nodes cache,
and drbd knows nothing about it yet...

> return (ie: the command won't complete) until the data is written to both 
> local and remote disks.
> Mark.
> -- 
> Mark Watts BSc RHCE MBCS
> Senior Systems Engineer
> QinetiQ Trusted Information Management
> Trusted Solutions and Services Group
> GPG Key: http://keyserver.veridis.com:11371/search?q=0x455420ED

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: Lars Ellenberg                            Tel +43-1-8178292-0  :
: LINBIT Information Technologies GmbH      Fax +43-1-8178292-82 :
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