High availability email server...

[Daniel Eckl]

Well, I don't have cluster knowledge, and so of course I simply believe 
you that a good cluster system will never have file locking problems.
I already stated this below!

But how will the cluster affect application level database locking? That 
was my primary question and you didn't name this at all.

A database file which is in use is practically always inconsistent until 
it's being closed by the database application.

That's why databases can be corrupt after an application crash and have 
to be reconstructed.

When you have two applications changing the same database file, you have 
a never ending fight, because every application thinks, the database is 
inconsistent, but it's just in use by another application. And every app 
will try to reconstruct it and so break it for the other app(s).

It's like letting two cyrus master run on the same single node! It will 
break in my opinion.

Can you shed some light on this subject?

Best,
Daniel

Dave McMurtrie schrieb:
> Daniel Eckl wrote:
> 
>> Hi Scott!
>>
>> Your statements cannot be correct by logical reasons.
>>
>> While on file locking level you are fully right, cyrus heavily depends 
>> on critical database access where you need application level database 
>> locking.
>>
>> As only one master process can lock the database, a second one either 
>> cannot lock the database or just crashes it with simultaneous write 
>> access. I didn't try it by myself for obvious reasons...
>>
>> If that didn't occur to you, then you had incredible luck, that there 
>> was no situation where both processes wanted to change the same db 
>> file simultaneously.
> 
> Hi Daniel,
> 
> Scott is not just lucky, he's using clustering technology that works.  
> When using a cluster filesystem that works, the locking semantics across 
> cluster nodes will be the same as those on a single node filesystem.  
> What you say above is simply not correct.
> 
> University of Pittsburgh is also running a 4-node active/active cluster 
> using Veritas Cluster Filesystem and it works very well.  The 
> performance is incredible, and as Scott pointed out you don't need the 
> complexity of murder or application-level replication.  Using a cluster 
> instead of Cyrus murder gives you both scalability and redundancy.  The 
> big tradeoff is that Veritas Cluster Filesystem costs money, while Cyrus 
> does not.
> 
> Thanks,
> 
> Dave
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