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Re: SSD + RAID

From: Greg Smith <greg(at)2ndquadrant(dot)com>
To: Karl Denninger <karl(at)denninger(dot)net>
Cc: Laszlo Nagy <gandalf(at)shopzeus(dot)com>, pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: SSD + RAID
Date: 2009-11-13 18:07:39
Message-ID: 4AFDA06B.1080704@2ndquadrant.com (view raw or flat)
Thread:
Lists: pgsql-performance
Karl Denninger wrote:
> If power is "unexpectedly" removed from the system, this is true.  But
> the caches on the SSD controllers are BUFFERS.  An operating system
> crash does not disrupt the data in them or cause corruption.  An
> unexpected disconnection of the power source from the drive (due to
> unplugging it or a power supply failure for whatever reason) is a
> different matter.
>   
As standard operating procedure, I regularly get something writing heavy 
to the database on hardware I'm suspicious of and power the box off 
hard.  If at any time I suffer database corruption from this, the 
hardware is unsuitable for database use; that should never happen.  This 
is what I mean when I say something meets the mythical "enterprise" 
quality.  Companies whose data is worth something can't operate in a 
situation where money has been exchanged because a database commit was 
recorded, only to lose that commit just because somebody tripped over 
the power cord and it was in the buffer rather than on permanent disk.  
That's just not acceptable, and the even bigger danger of the database 
perhaps not coming up altogether even after such a tiny disaster is also 
very real with a volatile write cache.

> With the write cache off on these disks they still are huge wins for
> very-heavy-read applications, which many are.
Very read-heavy applications would do better to buy a ton of RAM instead 
and just make sure they populate from permanent media (say by reading 
everything in early at sequential rates to prime the cache).  There is 
an extremely narrow use-case where SSDs are the right technology, and 
it's only in a subset even of read-heavy apps where they make sense.

-- 
Greg Smith    2ndQuadrant   Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com  www.2ndQuadrant.com


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