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Re: Battery Backed Cache for RAID

From: John A Meinel <john(at)arbash-meinel(dot)com>
To: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Cc: "Jeffrey W(dot) Baker" <jwbaker(at)acm(dot)org>,Peter Darley <pdarley(at)kinesis-cem(dot)com>,Pgsql-Performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Battery Backed Cache for RAID
Date: 2005-09-14 20:40:25
Message-ID: 43288AB9.4000204@arbash-meinel.com (view raw or flat)
Thread:
Lists: pgsql-performance
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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