Message-ID: <4C617C80.9080509@2ndquadrant.com>
Date: Tue, 10 Aug 2010 12:21:20 -0400
From: Greg Smith <greg@2ndquadrant.com>
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To: Scott Carey <scott@richrelevance.com>
CC: Michael March <mmarch@gmail.com>,
	"pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Completely un-tuned Postgresql benchmark results: SSD
	vs desktop HDD
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	<BAE01AAD-6C69-48C7-BD2E-77E463D49903@richrelevance.com>
	<AANLkTi=km+H+mZ=yyx0_HTFZ-yU2sh85D+p5cs+ZcUSF@mail.gmail.com>
	<A1640F99-A7C5-4F04-9A25-782C46BE7DAB@richrelevance.com>
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Scott Carey wrote:
> Also, the amount of data at risk in a power loss varies between 
> drives.  For Intel's drives, its a small chunk of data ( < 256K).  For 
> some other drives, the cache can be over 30MB of outstanding writes.
> For some workloads this is acceptable

No, it isn't ever acceptable.  You can expect the type of data loss you 
get when a cache fails to honor write flush calls results in 
catastrophic database corruption.  It's not "I lost the last few 
seconds"; it's "the database is corrupted and won't start" after a 
crash.  This is why we pound on this topic on this list.  A SSD that 
fails to honor flush requests is completely worthless for anything other 
than toy databases.  You can expect significant work to recover any 
portion of your data after the first unexpected power loss under heavy 
write load in this environment, during which you're down.  We do 
database corruption recovery at 2ndQuadrant; while I can't talk about 
the details of some recent incidents, I am not speaking theoretically 
when I warn about this.

Michael, I would suggest you read 
http://www.postgresql.org/docs/current/static/wal-reliability.html and 
link to it at the end of your article.  You are recommending that people 
consider a configuration that will result in their data being lost.  
That can be acceptable, if for example your data is possible to recreate 
from backups or the like.  But people should be extremely clear that 
trade-off is happening, and your blog post is not doing that yet.  Part 
of the reason for the bang per buck you're seeing here is that cheap 
SSDs are cheating.

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