[Linux-cluster] Re: E-Mail Cluster

[Rainer Duffner]

Nicholas Anderson wrote:
> Hi again all .....
>
> I guess i'm starting to understand how the things should work ....
>
> I was reading about GFS and all the documents that i found suppose 
> that you have a storage with a SAN and 2 or more machines connected 
> through FC to the SAN.
> Well, it seems to me that in this case the storage or the SAN switch 
> still being one single-point-of-failure right? If the storage or SAN 
> goes down, the whole service will be offline right ?

First of all, you (should) have redundant FC-switches (mulipathing).
Then, your storage has (should have) multiple controllers. Eg. HP EVA 
series.
If that isn't enough, there are solution to mirror the storage at the 
hardware-level.
Usually, this is in the 
"if-you-have-to-ask-it's-probably-too-expensive-for-you-anyway"-pricerange 
and thus only used where the (lack of) downtime is worth the investment.


>
> I thought that with GFS i could do something like a "Parallel FS" 
> where 2 (or more) machines would have the same data in their disks, 
> but this data would be synchronized in realtime ....
> am i totally noob or there really has a way to make FS's work in 
> parallel, synchronizing in realtime?
> I'd like to do this without having a SAN (cause i don't have one :-) 
> and i have only 1 storage ) and without leaving a 
> single-point-of-failure.
>
> Let me try to explain exactly what I'm thinking ...
>
> 3 servers, each one with a 300GB SCSI disk (local, no FC) to be 
> synchronized with the others (through GFS?? mounted and shared as a 
> /data f.ex.), and one 36GB disk only for the SO.
> All the servers would have smtp(sendmail with spamassassin and 
> clamav), imap and pop3 services running, and probably a squirrelmail.
>


You can have a master/slave solution with DRBD.


> Is it possible to do this? Is it possible to get this data 
> synchronized in realtime ?


I don't think so.
Well, Google has sort-of a solution via their "Google Filesystem". But 
not for you or me. :-(


>
> Thanks again for your really really important answers, and sorry for 
> asking so much noob questions :-)
>


IMO, hardware is very reliable these days (if you choose wisely). Things 
like DRBD seem (to me) only useful in very special cases - and I would 
fear that DRBD might create more problems than it solves.
In your special case (email), if you can't afford a SAN, get a used 
NetApp and store the maildirs there (qmail-style maildirs). Then 
NFS-mount them on the "cluster-nodes".
The NetApp is reliable enough for these scenarios and depending on the 
exact model, already contains a lot of redundancy in itself.





cheers,
Rainer