[Csync2] cysnc2 vs linking hackery

[Rafiu Fakunle]

Hi Johan,

Johan De Meersman wrote:

> CSync2, and rsync and whatever for that matter, are time-driven: they
> get called at certain times, regardless of the state of the data, and
> copy the data as it is at that point. Crontab can't go lower than
> 1-minute intervals, and if you've got lots of data, that may not even be
> enough time to copy everything.
> 
> Metadata, for most applications, is runtime data that changes every time
> you move the mouse, so to speak, and if you want to failover the service
> on-the-fly to another host, you need access to the most recent version
> of it. Thus, to replicate this, you need data-driven replication,
> something that watches the actual data and copies everything over as
> soon as it changes.
> 

I thought that was what Csync2 did. It's obvious to me now that it would
be highly inappropriate for what I want to do.

> DRBD's driver sticks it's feelers into the actual data stream of the
> block devices, and thus does exactly that: all data that passes through
> to disk gets copied out to the remote system.
> 
> So the answer is no, you can't. Not with CSync2. While, theoretically,
> it's quite possible to hack a filesystem driver to wake up when certain
> files are modified and pump them over to some other host, afaik it
> hasn't been done and your best guess is still the DRBD metadata volume
> with symlinked files.
> 
> Then again... If you're *really* desperate to get rid of that setup, 

I am :) I think it's very "unpretty".

you
> could have a look at FAM, the File Alteration Monitor. I've never really
> played with it, but it might be possible to hack up something that's
> close enough to data-driven replication that it suits the purpose.


Thanks for the detailed explanation. I'd looked at FAM and Gamin but I
was under the false impression that Csync2 was driven by inotify.

Back to the drawing board :)

Thx,

R.