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Re: Making the most of memory?

From: Greg Smith <gsmith(at)gregsmith(dot)com>
To: Guy Rouillier <guyr-ml1(at)burntmail(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Making the most of memory?
Date: 2008-01-24 00:54:24
Message-ID: Pine.GSO.4.64.0801231937090.12679@westnet.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Wed, 23 Jan 2008, Guy Rouillier wrote:

> Flash has a limited number of writes before it becomes unreliable.  On 
> good quality consumer grade, that's about 300,000 writes, while on 
> industrial grade it's about 10 times that.

The main advance that's made SSD practical given the write cycle 
limitation is increasing sophisticated wear leveling: 
http://en.wikipedia.org/wiki/Wear_levelling

The best devices now use static wear levelling; overviews at 
http://en.wikipedia.org/wiki/Static_Wear_Leveling and 
http://www.storagesearch.com/siliconsys-art1.html

The basic idea is that the number of writes to each block is tracked, and 
as it approaches the limit that block gets swapped with one that has been 
more read-only.  So essentially the number of writes before failure 
approaches something closer to 1M x number of blocks.  This means that as 
the size of the device goes up, so does its longevity.  If you believe the 
hype, the combination in the increase in size of designs with these more 
sophisticated wear-levelling approaches has now crossed the line where 
it's more likely a standard moving-parts hard drive will fail first if you 
compare it to a similarly sized SDD doing the same job (a standard 
mechanical drive under heavy write load also wears out faster than one 
doing less work).

--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD

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