From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mark Lord Subject: Re: SAS v SATA interface performance Date: Sat, 01 Dec 2007 22:45:09 -0500 Message-ID: <47522A45.50706@rtr.ca> References: <47506237.3000406@clear.net.nz> <47507F9A.3080109@msgid.tls.msk.ru> <20071130231705.51e056f4@the-village.bc.nu> <475110A8.5090709@clear.net.nz> <87f94c370712010637o5efa9c4rd9a86ec5a3cba6ff@mail.gmail.com> <4751B3DA.6030508@clear.net.nz> <4751BDA4.9080102@rtr.ca> <4751C6DA.3080409@garzik.org> <4751F482.60204@clear.net.nz> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from rtr.ca ([76.10.145.34]:1527 "EHLO mail.rtr.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753941AbXLBDpL (ORCPT ); Sat, 1 Dec 2007 22:45:11 -0500 In-Reply-To: <4751F482.60204@clear.net.nz> Sender: linux-ide-owner@vger.kernel.org List-Id: linux-ide@vger.kernel.org To: Richard Scobie Cc: Jeff Garzik , linux-ide@vger.kernel.org Richard Scobie wrote: > > > Jeff Garzik wrote: >> Mark Lord wrote: >>> SATA port multipliers (think, "hub") permit multiple drives >>> to be active simultaneously. >> >> Quite true, although the host controller could artificially limit >> this, giving the user a mistaken impression of their port multiplier >> being limited to one-command-per-N-drives. > > Interesting. I was basing my comments on what may well be a vested > interest slanted paper - see the sidebar on page 2. > > http://www.xtore.com/Downloads/WhitePapers/SAS_SATAValue%20Whitepaper_final.pdf > > > For the modest extra cost of a non-RAID SAS HBA and JBOD enclosure with > SATA drives, over a port multiplied setup, there would seem to be some > advantages. > > Or have I been taken in by the hype... :) .. Here's the "hype" part from that biased paper: > > Performance: Port Multipliers only support one active host > connection at a time, signicantly degrading e ective > throughput. Each time communication is initiated with a drive > time-consuming drive reset must occur. > > Data Integrity: PMs must close the connection to one drive > to open a new one to another. When a connection is closed > drive history (e.g., data source, destination drive, data & > command context) is lost; with each opened connection the > chance of misidentification and sending data to the wrong > drive is increased. Fiction. Or rather, heavily biased. Modern SATA hosts and PMs have no such issues. The key SATA term to ask for is "FIS-based switching". The biggest difference between SATA and SAS, is the same one we previously had between ATA and SCSI: Vendors like to position SAS/SCSI as a "premium" brand, and therefore cripple SATA/ATA with lower spin-rates (7200rpm max, or 10000rpm for WD Raptors, vs. 20000rpm for high end SAS/SCSI). There may be other firmware algorithm differences as well, but "RAID edition" SATA/ATA drives have similar low-readahead and fast-seek programming as their SAS/SCSI counterparts. Simple spin-rate (RPM) is the most significant distinguishing factor in nearly all scenarios. SAS/SCSI may also still win when connecting a ridiculously large number of drives to a single port. Cheers