[Linux-cluster] GNBD & Network Outage

Nigel Jewell nigel.jewell at pixexcel.co.uk
Tue Jan 25 10:43:36 UTC 2005

Dear all,

We've been looking at the issues of using GNBD to provide access to a 
block device on a secondary installation and we've hit a brick wall.  I 
was wondering if anyone had seen the same behaviour

On host "A" we do:

    gnbd_export -d /dev/sda2 -e foo -c

On host "B" we do:

    gnbd_import -i A

... and as you would expect /dev/gnbd/foo appears on B and is usable.

We have no other aspects of GFS in use.

Now - in order for this to be useful, we've been testing the effects of 
using GNBD if there is a LAN outage.  If we write a big file to a 
mounted file system on B:/dev/gnbd/foo and pull out the LAN cable 
halfway through the data being synced to A, host B never gives up trying 
to contact A.  In fact, if you plug in the cable 10 minutes later the 
sync recovers.

Now - on the surface - this doesn't seem like a big problem, but it is 
when you try and use the imported device alongside software RAID or when 
you want to do something "normal" like reboot the box.  Rebooting just 
stops when it trys to unmount the file systems.

We want to use B:/dev/gndb/foo alongside a local partition on B and 
create a RAID-1 using mdadm.  In the same scenario (where the LAN cable 
is pulled), the md device on B completely stops all of the IO on the 
machine because (presumably) the md software is trying to write to the 
gnbd device ... which is forever trying to contact host A ... and of 
course never gives up.  It would be nice if it did give up and the md 
software continued the md device in degraded mode.

So the question is this (got there in the end).  Can anyone suggest a 
solution and/or alternative/workaround?  Is it possible to specify a 
time-out for the GNBD import/export for when the LAN does die?

Any ideas?



PixExcel Limited
URL: http://www.pixexcel.co.uk
MSN: nigel.jewell at pixexcel.co.uk

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