[DRBD-user] Behaviour of drbd-secondary

Filip Sergeys filip.sergeys at verzekeringen.be
Tue Mar 22 17:48:15 CET 2005

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


Maybe my input can help you convince. Indeed mounting read only does
come in very handy for database logfiles.

This is the situation:
A masterdatabase which is online
A slavedatabase which is in standby

In normal operations:
The masterdatabase writes duplexed log files and archived log files to
the primary drbd device. 
At night the slavedatabase "wakes up" mounts the secondary drbd device
read only and applies the past 24 hours of changes captured in the
archived and the online redo log files. When done, the database goes
back to standby and the device is umounted.
To be more precise, in reality, I mount the device, copy the changes
since last night to a local device, unmount, and then work with the
local copies. The reason? I don't know how the slavedatabase would react
if the online redologfiles changed will rolling forward. 
So far this could also be done with NFS

When a failure happens on the masterdatabase and it goes down:
the slavedatabase kicks in (via heartbeat) mounts the drbd device in
primary, applies the changes since midnight and fires up in online
This could not be done with NFS because you wouldn't have access
anymore. rsync is not an option either because you need the committed
changes up to the very last second.

I think this is a very common setup and drbd is a magnificent tool to
reach the goal for this.  
So hope this is convincing enough ;)


Filip Sergeys

On Tue, 2005-03-22 at 17:05, Philipp Reisner wrote:

    Am Dienstag, 22. März 2005 14:53 schrieb Reine Larsson:
    > Hi,
    > After uplifting from 0.6.12 to 0.7.6 we discover a change in behaviour.
    > When did this change happen?
    from 0.6 -> 0.7
    > Is there something that can be done about it?
    Well, since users kept on mounting it read only on the secondary
    and Linux-2.6.x allows us to prohibit read accesses, we decided
    to do so.
      There are Filesystem out there that may crash the kernel when 
      mounted on a self-changing device ;)
    Short answer: no
    Long answer:  
     Just convince me that it I want this feature as well,
     and it might come back (as runtime configuration)
     in 0.7.11.
    : Dipl-Ing Philipp Reisner                      Tel +43-1-8178292-50 :
    : LINBIT Information Technologies GmbH          Fax +43-1-8178292-82 :
    : Schönbrunnerstr 244, 1120 Vienna, Austria    http://www.linbit.com :
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    drbd-user at lists.linbit.com

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