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

Mark Watts m.watts at eris.qinetiq.com
Thu May 10 11:44:58 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.

> 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.

   B: write IO is reported as completed, if it has reached
      local DISK and remote buffer cache.
      * for most cases.

   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 
return (ie: the command won't complete) until the data is written to both 
local and remote disks.


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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