Subject: Re: SAN vs Internal Disks
From: Mark Lewis <mark.lewis@mir3.com>
To: Harsh Azad <harsh.azad@gmail.com>
Cc: pgsql-performance@postgresql.org
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	<a199704d0709060958q5ebd6f81oe3562396a5c6040c@mail.gmail.com>
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Organization: MIR3, Inc.
Date: Thu, 06 Sep 2007 11:29:20 -0700
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On Thu, 2007-09-06 at 22:28 +0530, Harsh Azad wrote:
> Thanks Mark.
> 
> If I replicate a snapshot of Data and log files (basically the entire
> PG data directory) and I maintain same version of postgres on both
> servers, it should work right?
> 
> I am also thinking that having SAN storage will provide me with
> facility of keeping a warm standby DB. By just shutting one server
> down and starting the other mounting the same File system I should be
> able to bing my DB up when the primary inccurs a physical failure. 
> 
> I'm only considering SAN storage for this feature - has anyone ever
> used SAN for replication and warm standy-by on Postgres?
> 
> Regards,
> Harsh


We used to use a SAN for warm standby of a database, but with Oracle and
not PG.  It worked kinda sorta, except for occasional crashes due to
buggy drivers.

But after going through the exercise, we realized that we hadn't gained
anything over just doing master/slave replication between two servers,
except that it was more expensive, had a tendency to expose buggy
drivers, had a single point of failure in the SAN array, failover took
longer and we couldn't use the warm standby server to perform read-only
queries.  So we reverted back and just used the SAN as expensive DAS and
set up a separate box for DB replication.

So if that's the only reason you're considering a SAN, then I'd advise
you to spend the extra money on more DAS disks.

Maybe I'm jaded by past experiences, but the only real use case I can
see to justify a SAN for a database would be something like Oracle RAC,
but I'm not aware of any PG equivalent to that.

-- Mark Lewis