[ZODB-Dev] HA Setup and zodb index file of FileStorage

[Chris Withers]

Patrick Gerken wrote:
> system with ZEO for an ERP5 deployment. In my case I don't need to
> care for data replication, all is stored on a SAN considered HA by the
> customer already.

You run your live Data.fs off a SAN?
That usually makes for interesting performance problems in the best case!

> So my data.fs and index and all that stuff will already be available
> on my backup server.

...but better be very very careful that both the backup and live server 
dont' try and use the .fs and index files at the same time...

> The idea is that the backup server will watch the real ZEO, and start
> all services if the server goes down. 

How does it measure "down"?

> The thing which scared me to hell was the "rumour" that it can happen
> that the index can get corrupted and then everything has to be indexed
> again. 

without ZRS, don't even try to have a "live" backup. Best you can aim 
for is a half-hour swapover if you have a decent sized (10-20GB) 
database. With ZRS, it may be less, but I remain to be convinced.

> Given that writing a file and renaming a file can be considered
> atomic, and that no solar winds or similar things can screw up my
> filesystem, how can I screw up my index file?

The index checking is likely pessemistic, as you've noticed. 
Optimisations, I'm sure, would be welcome, but you'd need to prove they 
can't result in dangerous corruption...

cheers,

Chris

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