[ENBD] Load sharing with enbd?

[Peter T. Breuer]

"Also sprach Daniel Steen:"
> I'm looking into using enbd to keep two disks in sync (using enbd and
> fr1) both of which need to be able to be written to.
> How would this be done?

Well, assuming these are on different machines, the canonical
architecture would be to have the raid on a third front end machine
(which could be one of the existing two - or both, with failover) and
using enbd to allow the raid components to be (possibly) remote.

> Would it be neccesary to export a disk from
> machineA, and incorperate it in a raid array on machineB, and then
> export the raid disk (md0 or whatever) back to machineA? 

Well, I don't know why you would want to export it again ..  what was
wrong with keeping md0 on machineB?

> This does not
> seem like a very efficient solution.  Does anyone have a better method
> (or is enbd the wrong tool for this, and if so, what would be the
> correct one?)  My understanding is that it is not possible to simply
> export the base device from each machine, and incorperate that in a raid
> array on the other machine as we would get confused data due to disk
> caching.  Is this the case?  Would this be solved by mounting all
> devices (including the md devices) in "sync" mode?

It's not totally clear what you want to do. "keeping two disks in sync"
has many interpretations (including "keep both in a cupboard"). I get
the impression that you want to write to EITHER disk individually, and
have the changes magically appear on the other. That's not really
possible as stated (an old driver of mine called "yoke" did that).
Instead one usually agrees to write instead to a third device called
"md0" which transmits changes equally to both disks, but one doesn't
write directly to the disks.

Peter