Date: Fri, 07 Apr 2000 16:04:23 -0600 From: Warner Losh <imp@village.org> To: Bob.Gorichanaz@midata.com Cc: hackers@FreeBSD.ORG Subject: Re: bad memory patch? Message-ID: <200004072204.QAA02457@harmony.village.org> In-Reply-To: Your message of "Fri, 07 Apr 2000 07:46:24 CDT." <OF2F5C4FC5.C68B571C-ON862568BA.0045E942@midata.com> References: <OF2F5C4FC5.C68B571C-ON862568BA.0045E942@midata.com>
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In message <OF2F5C4FC5.C68B571C-ON862568BA.0045E942@midata.com> Bob.Gorichanaz@midata.com writes: : Maybe I'm mis-understanding something, but isn't this situation : analagous to bad sectors on a hard drive? Isn't this similar, at : least in theory, to remapping dead sectors and continuing to use the : drive? (except that the disk's onboard controller handles the : mapping instead of the OS) It is not analagous to the bad sectors on the hard drive. First, it is not always possible to detect a bad memory cell. In today's world, these cells are often bad only some of the time. They work unless pushed really hard in strange patters. They are just barely outside of spec, and usually work. This makes their detection hard. Second, in most modern memory, the general concensus is that if you have once cell that it is bad, others are sure to follow. Or that they have already followed and are still mostly working because they are only slightly out of spec. It is much harder to know with any degree of certainty the degree to which you can trust a memory stick with even one bad cell. Pretending to be able to do things which you aren't really doing is bad. It will lead to a lot of false negatives, which is why it is such an appolingly bad idea. With disk drives, the situation is easier. Either the sector writes or it doesn't. It is much rarer that the sector will be mostly good and only occasionally bad, although stories of such no doubt exist. In most modern drives, it isn't an issue anyway as the bad maps are kept around and bad block removal is automatically done at a layer below the OS. Warner To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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