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From: Greg Smith <greg@2ndQuadrant.com>
To: pgsql-performance@postgresql.org
Subject: Re: UPDATEDs slowing SELECTs in a fully cached database
Date: Sun, 17 Jul 2011 20:44:03 -0400
Message-ID: <4E2381D3.9020201@2ndQuadrant.com> (raw)
In-Reply-To: <CAMkU=1xFsFC+SR0LbqoJhkK=_KHynWo58YGMZSBdBtuQDzXxCQ@mail.gmail.com>
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	<4E20D988.3040007@yahoo.com>
	<CAMkU=1xFsFC+SR0LbqoJhkK=_KHynWo58YGMZSBdBtuQDzXxCQ@mail.gmail.com>

On 07/16/2011 06:33 PM, Jeff Janes wrote:
> 2.6 ms for an fsync seems awfully quick.  I wonder if EBS uses
> nonvolatile/battery-backed write cache, or if it just lies about fsync
> actually hitting disk.
>    

They have the right type of cache in there to make fsync quick, when you 
happen to be the lucky one to find it free of a write backlog.  So the 
best case is much better than a typical spinning drive with no such 
cache.  The worst case is in the 100ms+ range though on EBS.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD





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