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Date: Wed, 05 Oct 2005 11:33:49 -0400
From: Michael Stone <mstone+postgres@mathom.us>
Subject: Re: [HACKERS] A Better External Sort?
In-reply-to: <3E37B936B592014B978C4415F90D662DE11B55@MI8NYCMAIL06.Mi8.com>
To: Luke Lonergan <LLonergan@greenplum.com>
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On Wed, Oct 05, 2005 at 11:24:07AM -0400, Luke Lonergan wrote:
>Nope - it would be disk wait.

I said I/O overhead; i.e., it could be the overhead of calling the
kernel for I/O's. E.g., the following process is having I/O problems:

time dd if=/dev/sdc of=/dev/null bs=1 count=10000000                                                                       
10000000+0 records in                                                                                                      
10000000+0 records out                                                                                                     
10000000 bytes transferred in 8.887845 seconds (1125132 bytes/sec)                                                         
                                                                                                                           
real    0m8.889s                                                                                                           
user    0m0.877s                                                                                                           
sys     0m8.010s                                                                                                           

it's not in disk wait state (in fact the whole read was cached) but it's
only getting 1MB/s. 

Mike Stone