Message-ID: <4E553383.80106@boreham.org>
Date: Wed, 24 Aug 2011 11:23:15 -0600
From: David Boreham <david_list@boreham.org>
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To: pgsql-performance@postgresql.org
Subject: Re: Intel 320 SSD info
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On 8/24/2011 11:17 AM, Merlin Moncure wrote:
>
> hm, I think they need to reconcile those numbers with the ones on this
> page: http://www.intel.com/content/www/us/en/solid-state-drives/solid-state-drives-320-series.html
>
> 600 write ips vs 3.7k/23k.
>
>

They do provide an explanation (and what I find interesting about this 
document is that they are basically "coming clean" about the real 
worst-case performance, which I personally find refreshing and 
encouraging). The difference is that the high number is achieved if the 
drive does not need to perform a block erase to process the write (this 
is true most of the time since the capacity is over-provisioned and 
there is an expectation that GC will have generated free blocks in the 
background). The low number is the performance under worst-case 
conditions where the drive is a) full and b) no blocks have been 
trimmed, and c) GC wasn't able to run yet.

I suspect that in production use it will be possible to predict in 
advance when the drive is approaching the point where it will run out of 
free blocks, and hence perform poorly. Whether or not this is possible 
is a big question for us in planning our transition to SSDs in production.

Anyone using SSDs should be aware of how they work and the possible 
worst case performance. This article helps with that !