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Re: What exactly is postgres doing during INSERT/UPDATE ?

From: Aidan Van Dyk <aidan(at)highrise(dot)ca>
To: Merlin Moncure <mmoncure(at)gmail(dot)com>
Cc: Greg Stark <gsstark(at)mit(dot)edu>, Mark Mielke <mark(at)mark(dot)mielke(dot)cc>,Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>,Luke Koops <luke(dot)koops(at)entrust(dot)com>, Joseph S <jks(at)selectacast(dot)net>,"pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: What exactly is postgres doing during INSERT/UPDATE ?
Date: 2009-08-31 14:48:09
Message-ID: 20090831144809.GF12444@oak.highrise.ca (view raw or flat)
Thread:
Lists: pgsql-performance
* Merlin Moncure <mmoncure(at)gmail(dot)com> [090831 10:38]:
 
> I agree, that's good analysis.  The main point I was making was that
> if you have say a 10 disk raid 5, you don't involve 10 disks, only
> two...a very common misconception.  I made another mistake that you
> didn't catch: you need to read *both* the data drive and the parity
> drive before writing, not just the parity drive.
> 
> I wonder if flash SSD are a better fit for raid 5 since the reads are
> much cheaper than writes and there is no rotational latency.  (also,
> $/gb is different, and so are the failure cases).

The other thing that scares me about raid-5 is the write-hole, and the
possible delayed inconsistency that brings...

Again, hopefully mitigated by a dependable controller w/ BBU...

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

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