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Date: Sat, 22 Jun 2002 22:02:33 -0600 (MDT)
From: "Christopher E. Brown" <cbrown@woods.net>
To: "Griffiths, Richard A" <richard.a.griffiths@intel.com>
Cc: "'Andrew Morton'" <akpm@zip.com.au>, <mgross@unix-os.sc.intel.com>,
       "'Jens Axboe'" <axboe@suse.de>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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Subject: RE: ext3 performance bottleneck as the number of spindles gets large
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On Thu, 20 Jun 2002, Griffiths, Richard A wrote:

> I should have mentioned the throughput we saw on 4 adapters 6 drives was
> 126KB/s.  The max theoretical bus bandwith is 640MB/s.


This is *NOT* correct.  Assuming a 64bit 66Mhz PCI bus your MAX is
503MB/sec minus PCI overhead...

This of course assumes nothing else is using the PCI bus.


120 something MB/sec sounds a hell of a lot like topping out a 32bit
33Mhz PCI bus, but IIRC the earlier posting listed 39160 cards, PCI
64bit w/ backward compat to 32bit.

You do have *ALL* of these cards plugged into a full PCI 64bit/66Mhz
slot right?  Not plugging them into a 32bit/33Mhz slot?


32bit/33Mhz	(32 * 33,000,000) / (1024 * 1024 * 8) = 125.89 MByte/sec
64bit/33Mhz	(64 * 33,000,000) / (1024 * 1024 * 8) = 251.77 MByte/sec
64bit/66Mhz	(64 * 66,000,000) / (1024 * 1024 * 8) = 503.54 MByte/sec


NOTE: PCI transfer rates are often listed as

32bit/33Mhz, 132 MByte/sec
64bit/33Mhz, 264 MByte/sec
64bit/66Mhz, 528 MByte/sec

This is somewhat true, but only if we start with Mbit rates as used in
transmission rates (1,000,000 bits/sec) and work from there, instead
of 2^20 (1,048,576).  I will not argue about PCI 32bit/33Mhz being
1056Mbit, if talking about line rate, but when we are talking about
storage media and transfers to/from as measured by files remember to
convert.

-- 
I route, therefore you are.


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