From: Steinar H. Gunderson <sgunderson@bigfoot.com>
To: Luke Lonergan <llonergan@greenplum.com>
Cc: Jeffrey W. Baker <jwbaker@acm.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: Huge Data sets, simple queries
Date: Wed, 1 Feb 2006 02:26:15 +0100
Message-ID: <20060201012615.GA31927@uio.no> (raw)
In-Reply-To: <C0050CCE.1B604%llonergan@greenplum.com>
References: <1138737838.5648.4.camel@toonses.gghcwest.com>
<C0050CCE.1B604%llonergan@greenplum.com>
On Tue, Jan 31, 2006 at 12:47:10PM -0800, Luke Lonergan wrote:
>> Linux does balanced reads on software
>> mirrors. I'm not sure why you think this can't improve bandwidth. It
>> does improve streaming bandwidth as long as the platter STR is more than
>> the bus STR.
> ... Prove it.
FWIW, this is on Ultra160 disks (Seagate 10000rpm) on a dual Opteron running
Linux 2.6.14.3:
cassarossa:~# grep md1 /proc/mdstat
md1 : active raid1 sdf6[1] sda6[0]
cassarossa:~# dd if=/dev/sda6 of=/dev/null bs=8k count=400000
[system at about 35% wait for I/O and 15% system, according to top]
400000+0 records in
400000+0 records out
3276800000 bytes transferred in 54,488154 seconds (60137842 bytes/sec)
[system at about 45% wait for I/O and 7% system -- whoa?]
400000+0 records in
400000+0 records out
3276800000 bytes transferred in 52,523771 seconds (62386990 bytes/sec)
I'm not sure if it _refutes_ the assertion that the Linux RAID-1 driver can
do balancing of sequential reads, but it certainly doesn't present very much
evidence in that direction. BTW, sda and sdf are on different channels of a
dual-channel (onboard, connected via PCI-X) Adaptec board, so I doubt the bus
is the limiting factor.
/* Steinar */
--
Homepage: http://www.sesse.net/
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To: pgsql-performance@postgresql.org
Cc: sgunderson@bigfoot.com, llonergan@greenplum.com, jwbaker@acm.org
Subject: Re: Huge Data sets, simple queries
In-Reply-To: <20060201012615.GA31927@uio.no>
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