Date: Sat, 24 Oct 1998 07:24:19 -0500 From: David Kelly <dkelly@hiwaay.net> To: Don Lewis <Don.Lewis@tsc.tdk.com> Cc: freebsd-fs@FreeBSD.ORG, freebsd-scsi@FreeBSD.ORG Subject: Re: filesystem safety and SCSI disk write caching Message-ID: <199810241224.HAA19254@nospam.hiwaay.net> In-Reply-To: Message from Don Lewis <Don.Lewis@tsc.tdk.com> of "Thu, 22 Oct 1998 17:13:09 PDT." <199810230013.RAA19305@salsa.gv.tsc.tdk.com>
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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-fs" in the body of the message
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