[gentoo-user] Server Mirroring...

[gentoo-user] Server Mirroring...

[Mal Herring]

Hi Gentoo-User....

I run a Apache2 webserver and Postfix as per the virtual mailhost
guide...

For resilience I would like to have two gentoo boxes doing these tasks
for load balancing / redundancy - how can this be achieved?  Is there a
tool that will allow replication of the config files / mysql stuff to
allow the boxes to basically be a cluster ?

Oh - one more thing, the boxes will sit in different data centres !

Thanks in advance...

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Re: [gentoo-user] Server Mirroring...

[Sascha Lucas]

Hi Mal,

> I run a Apache2 webserver and Postfix as per the virtual mailhost
> guide...
>
> For resilience I would like to have two gentoo boxes doing these tasks
> for load balancing / redundancy - how can this be achieved?  Is there a
> tool that will allow replication of the config files / mysql stuff to
> allow the boxes to basically be a cluster ?

Thats not so easy to answer. Different solutions exists, each with pros 
and cons. It also depends heavily on the professional grad your solution 
should be.

You can solve this at different Levels: Hardware, OS, application. 
Hardware is expensive. OS far from perfect (i.e. openssi, openmosix). 
Applications must be cluster aware, moast aren't.

Do you mean http://www.gentoo.org/doc/en/virt-mail-howto.xml? If so you 
have to manage a bunch of apps: postfix, mysql, cyrus, apache.

Just some keywords:

easy with postfix: DNS round robin (solution inside DNS Server, nearly 
all MTAs are aware of this)

mysql: not "multi master" ready, just redundancy "master->slave", try 
postgres

apache: behaves in sens of "read only" -> LVS

cyrus: moast difficult part (shared storage/CFS,???) -> 
NAS|SAN+CFS+LVS+?dono?

Cluster/HA is a complex area. Perhaps you want a simple not so perfect 
solotion? Try periodicaly rsynced standby with manual switch over.

Sascha.
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Re: [gentoo-user] Server Mirroring...

[Arturo 'Buanzo' Busleiman]

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Sascha Lucas wrote:
> mysql: not "multi master" ready, just redundancy "master->slave", try
> postgres

Why not try MaxDB? Is not in portage yet, but see this description:

MaxDB is a re-branded and enhanced version of SAP DB, SAP AG's open source database. It is a
heavy-duty, SAP-certified open source database that offers high availability, scalability and a
comprehensive feature set. MaxDB complements the MySQL database server, targeted for large mySAP ERP
environments and other applications that require maximum enterprise-level database functionality. It
is available for a wide range of platforms, including Linux on several different architectures and
Windows.

http://freshmeat.net/projects/maxdb

- --
Arturo "Buanzo" Busleiman - www.buanzo.com.ar
Consultor en Seguridad Informatica / Dominio Digital TV - Da FOSS man!
KTP Consultores - info AT ktpconsultores.com.ar

Romper un sistema de seguridad los acerca tanto a ser hackers como el
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Re: [gentoo-user] Server Mirroring...

[kashani]

Arturo 'Buanzo' Busleiman wrote:
> Sascha Lucas wrote:
> 
>>mysql: not "multi master" ready, just redundancy "master->slave", try
>>postgres
>  
> Why not try MaxDB? Is not in portage yet, but see this description:
> 
> MaxDB is a re-branded and enhanced version of SAP DB, SAP AG's open source database. It is a
> heavy-duty, SAP-certified open source database that offers high availability, scalability and a
> comprehensive feature set. MaxDB complements the MySQL database server, targeted for large mySAP ERP
> environments and other applications that require maximum enterprise-level database functionality. It
> is available for a wide range of platforms, including Linux on several different architectures and
> Windows.
> 
> http://freshmeat.net/projects/maxdb
>

Can you use maxdb as the backend for the mail system? It looks like you 
can build support for MaxDB into PHP, but Postfix, Courier, Cyrus, etc 
may be more problematic.

kashani
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Re: [gentoo-user] Server Mirroring...

[Arturo 'Buanzo' Busleiman]

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kashani wrote:
> Can you use maxdb as the backend for the mail system? It looks like you
> can build support for MaxDB into PHP, but Postfix, Courier, Cyrus, etc
> may be more problematic.

It's mysql, actually. It should work easily, I guess.

- --
Arturo "Buanzo" Busleiman - www.buanzo.com.ar
Consultor en Seguridad Informatica / Dominio Digital TV - Da FOSS man!
KTP Consultores - info AT ktpconsultores.com.ar

Romper un sistema de seguridad los acerca tanto a ser hackers como el
encender autos puenteando los convierte en ingenieros automotrices.
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Re: [gentoo-user] Server Mirroring...

[kashani]

Arturo 'Buanzo' Busleiman wrote:
> kashani wrote:
> 
>>Can you use maxdb as the backend for the mail system? It looks like you
>>can build support for MaxDB into PHP, but Postfix, Courier, Cyrus, etc
>>may be more problematic.
> 
> 
> It's mysql, actually. It should work easily, I guess.

Er... well there is Mysql and Mysql Maxdb. My understanding is that 
maxdb is an entirely different code base. I can't see how it could be a 
drop in replacement for Mysql especially for binaries compiled against 
shared libs.

kashani
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Re: [gentoo-user] Server Mirroring...

[kashani]

Sascha Lucas wrote:
> Just some keywords:
> 
> easy with postfix: DNS round robin (solution inside DNS Server, nearly 
> all MTAs are aware of this)

So DNS round robins between the two mail servers. A new mail comes into 
server1, how does this mail make it to server2.

> mysql: not "multi master" ready, just redundancy "master->slave", try 
> postgres

I don't see that Postgres supports multiple masters either. Circular 
replication under Mysql may be a possibility if your db traffic isn't 
too high.

> apache: behaves in sens of "read only" -> LVS
> 
> cyrus: moast difficult part (shared storage/CFS,???) -> 
> NAS|SAN+CFS+LVS+?dono?

The storage has to reside somewhere. If that site goes down, both 
servers go down. You either need both servers in the same site with 
shared storage or figure out how to do a shared nothing backend.

kashani
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Re: [gentoo-user] Server Mirroring...

[Sascha Lucas]

>>  easy with postfix: DNS round robin (solution inside DNS Server, nearly all
>>  MTAs are aware of this)
>
> So DNS round robins between the two mail servers. A new mail comes into 
> server1, how does this mail make it to server2.

Yes this is definitely the difficultest part. It must be solved somewhere 
in the LDA (cyrus). The MTA just pass mails to the LDA. So round robin 
should work.

> I don't see that Postgres supports multiple masters either. Circular

I'm not an expert but what about this?
http://www.postgresql.org/about/news.289

> The storage has to reside somewhere. If that site goes down, both servers go 
> down. You either need both servers in the same site with shared storage or 
> figure out how to do a shared nothing backend.

It depends on how both sites are connected. Just like redundancy in 
Servers/Network you can have redundant storage. I.E. with (a)synchronous 
replication over IP or FC-networks.

BTW: I don't know a cluster-solution with cyrus. We use a cluster aware 
commercial one.

Sascha.
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Re: [gentoo-user] Server Mirroring...

[kashani]

Sascha Lucas wrote:
>>>  easy with postfix: DNS round robin (solution inside DNS Server, 
>>> nearly all
>>>  MTAs are aware of this)
>>
>> So DNS round robins between the two mail servers. A new mail comes 
>> into server1, how does this mail make it to server2.
> 
> Yes this is definitely the difficultest part. It must be solved 
> somewhere in the LDA (cyrus). The MTA just pass mails to the LDA. So 
> round robin should work.

Sure round robin works for incoming mail from other ISP's. But what 
about users trying to send mail that happens to get the DNS for the down 
server? Or trying to check your mail and hitting the down server? Or the 
server without your mail? DNS round robin is useful in many cases, but 
I'm not convinced it solves the problem we're looking at.

>> I don't see that Postgres supports multiple masters either. Circular
> 
> I'm not an expert but what about this?
> http://www.postgresql.org/about/news.289

Ah forgot about that package. Only saw the Slony stuff. However after 
thinking about it a bit, I don't see why you'd need multi-master. Can 
you elaborate?

>> The storage has to reside somewhere. If that site goes down, both 
>> servers go down. You either need both servers in the same site with 
>> shared storage or figure out how to do a shared nothing backend.
> 
> It depends on how both sites are connected. Just like redundancy in 
> Servers/Network you can have redundant storage. I.E. with (a)synchronous 
> replication over IP or FC-networks.

If you're replicating or copying to another site with completely 
different hardware then that fits the description of shared nothing. So 
either you're mounting the same storage or your mounting different 
storage in both locations. If you're mounting the same storage then one 
site can lose connection to it.

> BTW: I don't know a cluster-solution with cyrus. We use a cluster aware 
> commercial one.

Does the cluster aware stuff assume shared storage with both servers 
active/active or active/passive?

	I hate to be the guy shooting holes in everyone's ideas, but fault 
tolerant system design is a large part of what I've been doing for the 
past six years. It's not easy, it's not simple, and there are 254,861 
caveats to any plan based on the services you're supporting, network 
size, skill of the local admins, and a hundred other things. Part of the 
problem is that the original poster left the field wide open and we're 
all making a number of assumptions.
	The emails so far indicate that there is no perfect solution in this 
case. Any fault tolerant design is going to require some trade offs. The 
original poster needs to decide what's important. Is better uptime more 
important than throughput? Is cost a factor? Is data integrity more 
important than uptime?

kashani
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Re: [gentoo-user] Server Mirroring...

[Daniel da Veiga]

For the mail, I would use remote mounts for the directories that
matter, so, if your server goes down, you can script and/or use tools
to change the mount dir to another server that is kept rsynced (or
another similar tool) with the down server... This is easy job, you
just have to somehow monitor the activity and generate alerts about
failures (mostly I use scripts with cron jobs). Of course this would
require at least 3 machines and a fast LAN...

MySQL replication features combined with DNS would allow you to
rapidly exchange servers in case of failure, I have a mirror of my
mysql server replicating as slave, it can be turned into master while
I manage the actual master, syncing both is pretty easy and changing
master/slave status too.

--
Daniel da Veiga
Computer Operator - RS - Brazil
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