Re: endb 101

"Also sprach Christian Kujau:"
> i have 2 machines, located in 2 different hosting centers and each of them
> has a spare partition (hda3) for now. as i understand it each machine will

That's fine. 

> build an md0 out of hda3+nda0 (nda?).

Well, this depends on what you want. You CAN make a raid array out of
hda3 and nda0, yes.

> nda0 has to be "imported" [1] via
> enbd-client from another machine. if that is right, then i have this:
> 
> machine A: md0 [hda3+nda0]
>                        |
>                        |__________________________
>                                                   |
> machine B: md0 [hda3+nda0], enbd-server exports hda3

I don't understand this diagram very well, because of the "machine B:
md0 [hda3+nda0]" part. What is that doing there? machine B is exporting
hda3, not building it into a raid array!

> (and vice versa, so enbd-server from machine-A exports hda3)

No, not and vice-versa, I think!

> but that doesn't make sense to me, please correct me: each machine would
> export the partition, which itsself is part of an md0 array! or is the
> nda0 only a read-only device?

It doesn't make sense. Scrap the "machine B: md0 [hda3+nda0]" part and
it will make sense.

> 
> if i'm going to use 3 (or more) nodes, where would machine-C import its
> nda0 from? machine-A or machine-B?

Nowhere - I'm not sure what you mean ... nda0 is part of a raid array,
md0 on A, so that's where it stays!

> i plan to use this whole thing for several applications, such as mailspool

The intention is key. I think you want to build a failover arrangement?
In that case, you are doing fine. Now you export md0 from A, and when A
breaks you fail over to exporting hda3 from B instead. If B breaks
instead you still have the hda3 part of the raid on A working.

There are prepared failover scripts for heartbeat in the etc/ directory
in the archive.

> and webspace but perhaps for the database too. i think mailspool and
> webspace are "ok", but does anybody has experiences with a mysql database
> storing its tables on an enbd device? is this possible?

It's fine - it's even convenient, as you can switch off kernel
buffering (use the switch that does open O_DIRECT on the server) and
rely on the db servers to sync between distributed copies of the db.

Peter