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

Greg Freemyer greg.freemyer at gmail.com
Tue Oct 21 23:36:03 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 4:41 PM, Artur (eBoundHost)
<artur at eboundhost.com> 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.
> --

I don't think there is anything fundamentally wrong with your solution, but...

What happens when the "target" system fails.  If you want
high-availability, you need to implement a failover cluster for it.
So now you have 2 servers on the frontend and 2 storage servers on the
backend.  I have an idea, lets combine the front-end and back-end
servers!!!  We could use this really cool technology called drbd!!!

FYI: I suspect the other big issue is what happens in your model in
the presence of a network failure, even if it is relatively short.  I
believe a typical RAID setup would immediately fail the iSCSI drive,
and then when it came back online it would require a very long slow
rebuild.  Drbd is much more graceful in that it maintains a bitmap of
out of date blocks and when the link comes back up, only those blocks
are sync'ed.

Greg Freemyer
Litigation Triage Solutions Specialist
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The Norcross Group
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