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Path: euryale.cc.adfa.oz.au!newshost.carno.net.au!harbinger.cc.monash.edu.au!nntp.coast.net!news.sgi.com!www.nntp.primenet.com!nntp.primenet.com!cpk-news-hub1.bbnplanet.com!newsfeed.internetmci.com!in2.uu.net!munnari.OZ.AU!metro!metro!asstdc.scgt.oz.au!nsw.news.telstra.net!elausrv2.att.net.au!surya From: Richard Laxton <richard@real.net.au> Newsgroups: comp.unix.bsd.freebsd.misc,comp.unix.bsd.bsdi.misc Subject: Re: Why one should buy parity memory for reliability? Date: Wed, 25 Sep 1996 15:54:28 +1000 Organization: RealNet Access Lines: 53 Message-ID: <3248C914.3C17@real.net.au> References: <32485B0D.41C6@austin.ibm.com> Reply-To: richard@real.net.au NNTP-Posting-Host: 203.17.240.113 Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Mailer: Mozilla 3.0 (WinNT; I) Xref: euryale.cc.adfa.oz.au comp.unix.bsd.freebsd.misc:27929 comp.unix.bsd.bsdi.misc:4969 Given that parity checking can only *detect* errors, one might say that there is no point. However memory corruption can occur for many reasons, even something like radioactive decay can change the state of memory cells and there is a chance that that change could cause chain reactions within the system if the change were to the right place (e.g. jump tables or whatever). Parity errors generally cause a NMI (non maskable interrupt) that may halt the system. Given that most UNIX servers up up for between 7 and 700 days at a time the chances of some memory corruption are quite good. There are newer motherboards made by DFI and others that do ECC with normal parity checked SIMMs. This could be useful in that the system will be able to recover transparently from any memory errors and potentially stay up much longer. Just a few thoughts... Richard. Tushar Patel wrote: > > Hi, > > I was going through the FreeBSD hadbook and one of the things suggested > in the book is buy "parity memory". > > I was involve in designing one of the microcontroller for Motorola, > one of the things we left out for the future controller was not > supporting parity (less pin count). The reasoning was, In the normal > condition the error should not happen, if it does then there is > somethings seriouly wrong, like having high temprature or more noise > in the system. So, the bord designer better design the system correcly. > (They had some hard numbers to back the theory and this is just the > brif explanation). > > If the Board supports the parity memory and error occurs then in > theory the OS should be notified and the access should be reapeted. > > Does FreeBSD support such error conditions? > > What happens in the case of the DMA transfer from the DISK to the > memory or from memory to disk, if the memory error occures then > processor is not looking at the data bus, so does that mean that the > DMA master (SCSI controller) will detect the parity error and > retransfer the data? > > There is a big difference in the price between the parity and non > parity memory so I am trying to justify the parity memory purchase. > > Please make comments. > > Thanks, > Tushar