[8213] in linux-scsi channel archive
Re: scatter gather or not to scatter gather
daemon@ATHENA.MIT.EDU (Douglas Gilbert)
Thu Feb 24 13:38:06 2000
Message-ID: <38B53B55.247A6C51@interlog.com>
Date: Thu, 24 Feb 2000 09:08:21 -0500
From: Douglas Gilbert <dgilbert@interlog.com>
MIME-Version: 1.0
To: Kurt Garloff <garloff@suse.de>
Cc: The Linux SCSI list <linux-scsi@vger.rutgers.edu>
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Kurt Garloff wrote:
>
> [snip]
>
> Which means a increase in wasted CPU cycles and worse performance ...
> The mid-layer will issue one SCSI command for every SG segment, unless it
> can do the clustering for you ...
>
> It might not be an issue for your device, but consider that while the data
> transfer gets faster and faster (up to 160MB/s with DDR LVD Wide SCSI), the
> time needed to arbitrate the bus, select, transfer the command, the messages
> and the status, did rarely get faster, so you really want to keep the
> number of commands needed to a minimum.
Kurt,
This url is to a white paper by IBM about "packetized" scsi.
http://www.storage.ibm.com/hardsoft/diskdrdl/library/whitepap/tech/hdwpacket.htm
It seems to address the current speed problems of the slow async (8 bit)
setup protocols in parallel scsi in a backward compatible way. IBM
are already marketing what they classify as U160+ disks which include
this feature. It will most likely be standardized when U320 and
U640 arrive.
Does anybody make a U160+ controller?
Doug Gilbert
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