Heartbeat with dual SCSI config

[Alan Robertson]

Hi Roberto,

I've CCed this reply to the linux-ha mailing list.

Roberto Zini wrote:

> Hi Alan !
> 
> In the past few days I tried you heartbeat solution on a couple of
> Linux boxes and so far I was impressed by the results I got !


Thanks!


> I'm trying to use the HB (heartbeat) solution as to allow a given
> process (eg, Apache running several CGI scripts) to operate on a
> SCSI disk which is shared between the above 2 boxes. Just to provide
> you with some numbers, I'm using a couple of Adaptec 29160 HA (configured as 
> ID=14 on the first box and ID=13 on the second one) whose secondary bus is 
> connected to an external SCSI disk configured as ID=5 (the primary SCSI
> bus is being used for the boot/root disk).
> 
> Let me preface that I'm neither a SCSI expert nor a Linux one but in
> my tests I've seen that both boxes are able to access the same 
> shared HD (which has been prepared with a Linux partition) at the same
> time (ie, they can "mount" it without problems) which can lead to data
> corruption if both OSes try to write the same chunk of data.
> 
> I'm wondering if there is an HW/SW solution which prevents the "failover"
> box (from the HB point of view) from mounting the external disk when it's 
> already being mounted by the primary box.


You could write a resource script which removes the /dev entry when the 
other side has the disk mounted.  Or you could do the equivalent at the 
kernel level like this:
	echo "scsi-remove-single-device A 0 D 0" > /proc/scsi/scsi
to make the kernel believe the device is gone, and then also do
	echo "scsi-add-single-device A 0 D 0" > /proc/scsi/scsi
to make it come back just before takeover.

Maybe someone should write such a resource script...

In this case A is number of the SCSI adapter according to /proc/scsi/scsi, 
and D is the logical drive number (I think this is the SCSI target ID).

Thanks to the IBM ServeRAID team for this cool tip.

But, you should still use a STONITH device to *GUARANTEE* that the other 
machine isn't using the disk.  A STONITH device is a smart power switch. 
Heartbeat supports around 8-10 types of STONITH devices.

	-- Alan Robertson
	   alanr@unix.sh