Re: Oracle RAC with ENBD

"Also sprach roland:"
> > But the word "shared" makes me wonder! You can't in principle share
> > remote devices because every kernel involved would have a different
> > cached image of the device. They'd all be unaware of changes made by
> > the others if they were to areas they already had cached.
> yep - but didn`t the paper mention to make those devices RAW ?

That's the point. You can't allow the clients to cache, so you have to 
use raw i/o (direct i/o in modern kernels ... raw is a charcter device,
and you really don't want to be talking to a character device if you
can help it, direct i/o is much better). But that's an application-
dependent thing. The application has to decide to use a raw device
bound to the enbd device, or to do direct i/o to the enbd device
itself.

> Didn`t RAW mean something like "non cached" ?

It does. But it's old. Nowadays direct i/o is the thing.

Still, if you are talking about oracle, they might be able to deal with
character i/o! 

It' an application thing.

> > The only soluton is to not cache anything, something that can be done
> > by doing direct i/o on the server (the -y switch?).  But how do you turn
> > off caching on the client kernels?  You can't. All you can do is access
> > the device with direct i/o there too. So it's an application-dependent
> > thing.
> Do I really need to turn off caching on the server ?

No - you are right, you don't absolutely need direct i/o on the server.
But much of the idea in using no caching underneath oracle is to let
oracle do its own caching.  Therefore you should try and turn it off
serverside as well!  And it becomes necessary as soon as you have more
than one database node involved. Not that I know how or if you can.

> > But you CAN do it provided you promise to use direct i/o (OPEN_DIRECT)
> > on the clients at the application level and use direct i/o on the server
> > too (-y ?)
> Yes - Oracle does that OPEN_DIRECT afaik.

OK.

> > I don't know. You can tell me!
> hopefully soon :)
> 
> > Send money care of the elves :).
> :)
> 

Greetings from a freezing madrid.

Peter