Message-ID: <43288AB9.4000204@arbash-meinel.com>
Date: Wed, 14 Sep 2005 16:40:25 -0400
From: John A Meinel <john@arbash-meinel.com>
User-Agent: Mozilla Thunderbird 1.0.6 (Macintosh/20050716)
MIME-Version: 1.0
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: "Jeffrey W. Baker" <jwbaker@acm.org>,
	Peter Darley <pdarley@kinesis-cem.com>,
	Pgsql-Performance <pgsql-performance@postgresql.org>
Subject: Re: Battery Backed Cache for RAID
References: <PDEOIIFFBIAABMGNJAGPAELFDOAA.pdarley@kinesis-cem.com>
	<1126722523.27650.6.camel@toonses.gghcwest.com>
	<20050914200324.GB26714@surnet.cl>
In-Reply-To: <20050914200324.GB26714@surnet.cl>
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Alvaro Herrera wrote:
> On Wed, Sep 14, 2005 at 11:28:43AM -0700, Jeffrey W. Baker wrote:
>
>>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.
>
>
> Just curious: how long are the batteries supposed to last?
>

The recent *cheap* version of a ramdisk had battery backup for 16 hours.
(Very expensive ramdisks actually have enough battery power to power a
small hard-drive to dump the contents into).

I'm guessing for a RAID controller, the time would be in the max 1 day
range.

John
=:->


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