Message-ID: <4B8331D5.7000802@2ndquadrant.com>
Date: Mon, 22 Feb 2010 20:39:33 -0500
From: Greg Smith <greg@2ndquadrant.com>
User-Agent: Thunderbird 2.0.0.23 (X11/20090817)
MIME-Version: 1.0
To: Mark Mielke <mark@mark.mielke.cc>
CC: Arjen van der Meijden <acmmailing@tweakers.net>,
	pgsql-performance@postgresql.org
Subject: Re: SSD + RAID
References: <201002211410.o1LEAht05941@momjian.us>
	<4B814920.1040204@cheapcomplexdevices.com>
	<4B82187A.7060604@2ndquadrant.com> <4B822DFF.50501@tweakers.net>
	<4B8329A3.4040003@2ndquadrant.com>
	<4B832B3E.9080207@mark.mielke.cc>
In-Reply-To: <4B832B3E.9080207@mark.mielke.cc>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

Mark Mielke wrote:
> I had read the above when posted, and then looked up SRAM. SRAM seems 
> to suggest it will hold the data even after power loss, but only for a 
> period of time. As long as power can restore within a few minutes, it 
> seemed like this would be ok?

The normal type of RAM everyone uses is DRAM, which requires constrant 
"refresh" cycles to keep it working and is pretty power hungry as a 
result.  Power gone, data gone an instant later.

There is also Non-volatile SRAM that includes an integrated battery ( 
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2648 is a typical 
example), and that sort of design can be used to build the sort of 
battery-backed caches that RAID controllers provide.  If Intel's drives 
were built using a NV-SRAM implementation, I'd be a happy owner of one 
instead of a constant critic of their drives.

But regular old SRAM is still completely volatile and loses its contents 
very quickly after power fails.  I doubt you'd even get minutes of time 
before it's gone.  The ease at which data loss failures with these Intel 
drives continue to be duplicated in the field says their design isn't 
anywhere near good enough to be considered non-volatile.

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us