From owner-freebsd-scsi Sat Oct 24 05:54:07 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id FAA09656 for freebsd-scsi-outgoing; Sat, 24 Oct 1998 05:54:07 -0700 (PDT) (envelope-from owner-freebsd-scsi@FreeBSD.ORG) Received: from mail.HiWAAY.net (fly.HiWAAY.net [208.147.154.56]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id FAA09640; Sat, 24 Oct 1998 05:54:04 -0700 (PDT) (envelope-from dkelly@n4hhe.ampr.org) Received: from nospam.hiwaay.net (tnt3-227.HiWAAY.net [208.147.146.227]) by mail.HiWAAY.net (8.9.0/8.9.0) with ESMTP id HAA19730; Sat, 24 Oct 1998 07:53:29 -0500 (CDT) Received: from n4hhe.ampr.org (localhost.ampr.org [127.0.0.1]) by nospam.hiwaay.net (8.8.8/8.8.8) with ESMTP id HAA19254; Sat, 24 Oct 1998 07:24:19 -0500 (CDT) (envelope-from dkelly@n4hhe.ampr.org) Message-Id: <199810241224.HAA19254@nospam.hiwaay.net> X-Mailer: exmh version 2.0.2 2/24/98 To: Don Lewis cc: freebsd-fs@FreeBSD.ORG, freebsd-scsi@FreeBSD.ORG From: David Kelly Subject: Re: filesystem safety and SCSI disk write caching In-reply-to: Message from Don Lewis of "Thu, 22 Oct 1998 17:13:09 PDT." <199810230013.RAA19305@salsa.gv.tsc.tdk.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Date: Sat, 24 Oct 1998 07:24:19 -0500 Sender: owner-freebsd-scsi@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.org Don Lewis writes: > On Oct 16, 6:09am, David Kelly wrote: > } Subject: Re: filesystem safety and SCSI disk write caching > > } Why not? It might be interesting to put a recording voltmeter such as a > } digital storage oscilloscope on the HD power leads when Don is punching > } the reset. No telling what kind of voltage surges are generated when > } the load on the power supply is altered. > > Ok, so the chapter in the handbook about SCSI write caching will recommend > connecting a recording voltmeter to the power supply and monitoring it > under varying load (including when the reset button is hit) to make sure > there are no problems before enabling write caching? Repeat this procedure > after adding new hardware and periodically as the power supply capacitors > age. > > And you still can't prove that you don't have a power supply problem > lurking, since you can't prove a negative. All you can state is that > the power looks clean under the conditions that you tested. Problems > still might occur when the machine is placed in service. I do not suggest use of a recording voltmeter or storage oscilloscope be mentioned in the handbook. My point is that in this day of generic PC parts the quality control aspect is getting skipped. While an individual PS may be tested for UL compliance, and the MB for FCC emissions, the system as a package gets skipped. The Mom & Pop PC Shop doesn't have a clue other than, "We sold 100 systems this month and you are the only one complaining." Testing for FCC emissions levels and UL safety on entire systems are nil. For data loss on reset you can prove a negative if you can reproduce the failure during your measurements. If you sample the voltage often enough thru the event then you can prove there was not a slower voltage spike causing the problem. My 50 MHz DSO says it samples at 100 MHz. Its much harder to monitor current as your power leads would have to be cut or some other inline calibrated very low value resistor inserted. But to do a complete job current should be monitored also. Current measurements will tell you if the load on the PS is changing. If current doesn't change significantly thru a RESET, then this is not a boundary condition. Problems are usually found at the boundaries. My suggestion of monitoring the power supply came from the nature of this list and its participants where skills and tools are above average. And the result is a product which is well above average and suitable for use by those who never give it a second thought. A handbook entry on SCSI caching is a attempt to cause such as second thought in more than would have in the first place. -- David Kelly N4HHE, dkelly@nospam.hiwaay.net ===================================================================== The human mind ordinarily operates at only ten percent of its capacity -- the rest is overhead for the operating system. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message