Subject: Re: Battery Backed Cache for RAID
From: "Jeffrey W. Baker" <jwbaker@acm.org>
To: Peter Darley <pdarley@kinesis-cem.com>
Cc: Pgsql-Performance <pgsql-performance@postgresql.org>
In-Reply-To: <PDEOIIFFBIAABMGNJAGPAELFDOAA.pdarley@kinesis-cem.com>
References: <PDEOIIFFBIAABMGNJAGPAELFDOAA.pdarley@kinesis-cem.com>
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Date: Wed, 14 Sep 2005 11:28:43 -0700
Message-Id: <1126722523.27650.6.camel@toonses.gghcwest.com>
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On Wed, 2005-09-14 at 11:25 -0700, Peter Darley wrote:
> 	I'm getting a new server for our database, and I have a quick question
> about RAID controllers with a battery backed cache.  I understand that the
> cache will allow the cache to be written out if the power fails to the box,
> which allows it to report a write as committed safely when it's not actually
> committed.

Actually the cache will just hold its contents while the power is out.
When the power is restored, the RAID controller will complete the writes
to disk.  If the battery does not last through the outage, the data is
lost.

> 	My question is, if the power goes off, and the drives stop, how does the
> battery backed cache save things out to the dead drives?  Is there another
> component that is implied that will provide power to the drives that I
> should be looking into as well?

A UPS would allow you to do an orderly shutdown and write contents to
disk during a power failure.  However a UPS can be an extra point of
failure.

-jwb