Re: postgresql cluster on SAN

From: Chris Travers <chris(at)travelamericas(dot)com>
To: "Spiegelberg, Greg" <gspiegelberg(at)cranel(dot)com>,pgsql-admin(at)postgresql(dot)org
Subject: Re: postgresql cluster on SAN
Date: 2005-09-23 19:57:42
Message-ID: 43345E36.1020201@travelamericas.com
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Spiegelberg, Greg wrote:

>This is possible. Below is a brief recipe.
>
>1. Use Veritas Storage Foundation HA with Cluster File System.
>2. Have a volume with cfs available to 2 or more systems via SAN.
>3. Install PostgreSQL and be sure PGDATA is on the cfs mount
>4. Create a Storage Checkpoint of cfs mount (read-write virtual copy)
>5. Bring up PostgreSQL on node1
>6. Mount storage checkpoint on node2
>7. Start PostgreSQL on node2
>
>Drawbacks? Heck yes. There are many and you should understand the
>nature of Veritas Storage Checkpoints before using.
>
Drawback #1-- only works read-only on all but one node. And I am not
even sure what will happen when you start writing on that one node...

> I have tested it
>though in conjunction with dblink() and some other proprietary API's we
>use for load balancing and the potential is there to manage many large
>SELECT's.
>
>
Ok, but what happens when you start doing inserts, updates, and deletes.

Somehow I think that PostgreSQL's dependance on shmem and semaphores
will cause corruption in your database as soon as you start writing to
it on both nodes. And I don't know enough about PostgreSQL's internal
structure to know if writing on only one node is safe.

Best Wishes,
Chris Travers
Metatron Technology Consulting

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