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Re: performance on new linux box

From: Ben Chobot <bench(at)silentmedia(dot)com>
To: Scott Carey <scott(at)richrelevance(dot)com>
Cc: Craig James <craig_james(at)emolecules(dot)com>, "Timothy(dot)Noonan(at)emc(dot)com" <Timothy(dot)Noonan(at)emc(dot)com>, "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: performance on new linux box
Date: 2010-07-15 02:50:28
Message-ID: A8F65D92-00A9-433F-83C0-04DEB55D57DF@silentmedia.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Jul 14, 2010, at 6:57 PM, Scott Carey wrote:

> But none of this explains why a 4-disk raid 10 is slower than a 1 disk system.  If there is no write-back caching on the RAID, it should still be similar to the one disk setup.

Many raid controllers are smart enough to always turn off write caching on the drives, and also disable the feature on their own buffer without a BBU. Add a BBU, and the cache on the controller starts getting used, but *not* the cache on the drives.

Take away the controller, and most OS's by default enable the write cache on the drive. You can turn it off if you want, but if you know how to do that, then you're probably also the same kind of person that would have purchased a raid card with a BBU.

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