[Linux-HA] choosing the right setup

[Alan Robertson]

Alexander Opitz wrote:
> Hi,
> 
> I've a problem and I don't know how I should setup this, I hope someone can 
> help me.
> 
> I've 2 standard PC's with IDE they should work together in a nodegroupe (later 
> there should a 3rd pc added).
> 
> On both should run MaxDB first as primary and on the other as Hot Standby.
> 
> For this I need a shared partition (the primary writes in a file and the hot 
> standby reads it and updates his own database).
> 
> I've no SAN or other storage device only the IDE hard drives in both pc's.
> 
> How I can have this shared partition on both computers?
> 
> DRBD can do this, but the partition in the second computer can't access the 
> drive while it is secondary so the database can only be updated after 
> failover, but this needs to much time then.

DRBD takes over in milliseconds.  The filesystem mounts in seconds.  Does 
your application (MaxDB) take a long time to start after a crash?

> GNBD only exports a block device from the other computer, but if the secondary 
> fails while it is in standby the first computer can't write the file.
> 
> ODR is only in draft2 since some years and I don't found something better.
> 
> The mirroring of GFS seems to be in work/progress ... I found no documentation 
> for it.
> 
> My solution would be using local partition and a GNBD exported one and set 
> over it software raid (md device) and on top GFS ... but I think that can put 
> me in trouble while rebuild.

If you are going to share data between the two sides, then you're going to 
run into some kind of consistency check that has to happen when you fail 
over.  GFS has the same issue.  Your application has the same issue.

VERY few applications can operate in the mode you describe.  Most go crazy 
if someone under them modifies the data while they're running.


-- 
     Alan Robertson <alanr at unix.sh>

"Openness is the foundation and preservative of friendship...  Let me claim 
from you at all times your undisguised opinions." - William Wilberforce