From: Achilleas Mantzios <achill@matrix.gatewaynet.com>
Organization: Dynacom Tankers Mgmt
To: pgsql-admin@postgresql.org
Subject: Re: PostgreSQL in Shared Disk Failover mode on FreeBSD+CARP+RAIDZ
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Date: Tue, 21 Dec 2010 10:52:40 +0300
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=CE=A3=CF=84=CE=B9=CF=82 Tuesday 21 December 2010 03:23:25 =CE=BF/=CE=B7 sn=
oop@email.it =CE=AD=CE=B3=CF=81=CE=B1=CF=88=CE=B5:
> Hi everybody,
> I'm trying to figure out a way to setup a PostgreSQL HA cluster solution.
>=20
> I've done some research and I've an idea of different possible solutions,
> but I'd honestly like to implement it using CARP in a "Shared Disk Failov=
er"
> fashion.

This reminds me of the serialization stack on shared disks (DASD) in the MV=
S IBM oper system.
It takes a lot of work to do that on OS level. Its smth beyond multiprocess=
ing and high availability.
Think about it. In FreeBSD (or any of-the-shelf Unix), there is no inherent=
 way to implicitly lock files.
Even on the same machine, if two users/processes modify the same file, the =
one who saves (closes)
the file last, generally wins. In order to do that, you will have to use ex=
plicit locking done by the application.
The OS by itself does not do that by default.
Things get tougher in a networked environment since file serialization shou=
ld be applied at network-level.
This combination of data sharing (the idea of decoupling the concept of red=
undant hardware
from the concept of redundant disks) with the characteristics of parallel c=
omputing was realized in=20
IBM's Parallel Sysplex technology: http://en.wikipedia.org/wiki/IBM_Paralle=
l_Sysplex
On top of it, someone could have CICS, DB2,etc... almost any available appl=
ication.
It would be interesting to know if there is some concept close to it in the=
 open source-Unix world.

=2D-=20
Achilleas Mantzios