Date: Tue, 23 Feb 2010 15:34:35 -0500
From: Aidan Van Dyk <aidan@highrise.ca>
To: david@lang.hm
Cc: Ron Mayer <rm_pg@cheapcomplexdevices.com>,
	Bruce Momjian <bruce@momjian.us>, Greg Smith <greg@2ndquadrant.com>,
	pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: SSD + RAID
Message-ID: <20100223203435.GV14128@oak.highrise.ca>
References: <201002221437.o1MEbDJ13005@momjian.us>
	<4B82C642.8020409@cheapcomplexdevices.com>
	<alpine.DEB.2.00.1002230022110.27818@asgard.lang.hm>
	<alpine.DEB.2.00.1002231131300.5131@asgard.lang.hm>
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* david@lang.hm <david@lang.hm> [100223 15:05]:

> However, one thing that you do not get protection against with software  
> raid is the potential for the writes to hit some drives but not others. 
> If this happens the software raid cannot know what the correct contents 
> of the raid stripe are, and so you could loose everything in that stripe  
> (including contents of other files that are not being modified that  
> happened to be in the wrong place on the array)

That's for stripe-based raid.  Mirror sets like raid-1 should give you
either the old data, or the new data, both acceptable responses since
the fsync/barreir hasn't "completed".

Or have I missed another subtle interaction?

a.

-- 
Aidan Van Dyk                                             Create like a god,
aidan@highrise.ca                                       command like a king,
http://www.highrise.ca/                                   work like a slave.

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