[DRBD-user] iscsi + md0 = tell me why this is a bad idea

Lars Ellenberg lars.ellenberg at linbit.com
Tue Oct 21 23:35:27 CEST 2008

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


On Tue, Oct 21, 2008 at 03:41:46PM -0500, Artur (eBoundHost) wrote:
> We're getting ready to test a drbd + nfs system in production and i  
> started looking into iscsi.  Thought of something that i couldn't find  
> an answer to on google, maybe i'm not looking hard enough (not on the  
> first 5 results)
>
> Why not have 2 storage systems exporting iscsi and simply software md  
> raid the exports on a target system?  This would take out complexity of  
> drbd and heartbeat entirely, no?
>
> Only thing i can think against this setup is it pushes lots of bandwidth  
> to both storage servers, but assuming that network is already in place  
> this should not be a barrier.  And yes, it would require an additional  
> 1gb nic on the target server (1 from each storage system)
>
> I'm definitely not going with this setup because drbd works great, but  
> still looking for anything you can say about why my scenario would not  
> be a good idea.

think about what can go wrong.
and how to recover from that.

what if ...
  one target breaks
  one link breaks
  one initiator breaks

how to
  do a failover.
  do a failback.
  determine "better" (more recent) data.
  determine resync direction.
  determine which areas of the disks to sync.
  determine data divergence.

I think in all areas drbd does better than nbd/iscsi + md raid1,
but I am happy to hear all ideas, and use them as inspiration
for future linux storage replication solutions.

-- 
: Lars Ellenberg                
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com

DRBD® and LINBIT® are registered trademarks of LINBIT, Austria.
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