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On Wed, 9 Jan 2013 14:48:33 +0000 (UTC) |
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Grant Edwards <grant.b.edwards@×××××.com> wrote: |
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> On 2013-01-09, Alan McKinnon <alan.mckinnon@×××××.com> wrote: |
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> > On Wed, 9 Jan 2013 02:47:07 +0000 (UTC) |
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> |
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> > The data on a medium can corrupt, and it can corrupt silently for a |
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> > long time. |
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> |
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> And I'm saying I've never seen that happen. |
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> |
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> So you're saying that the data on a medium can corrupt without being |
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> detected by the block encodings and CRCs used by the disk controller? |
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No, I'm not saying that *at*all* |
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I've been saying all along that data which you never go near can |
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corrupt silently while you are not using it. When you do eventually get |
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around to reading it, the electronics will do what they are designed to |
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do and properly detect a problem that already happened. |
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> |
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> > At some point it may deteriorate to where it passes a cusp |
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> > and then you will get your first visible sign |
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> |
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> No, the first visible sign in the scenario you're describing would be |
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> a read returning erroneous data. |
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That's what I said. The first VISIBLE sign is an error. You want to |
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catch it before then. |
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Analogy time: A murderer plans to do Grant. By observing Grant and only |
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observing Grant, the first visible sign of an issue is the death of |
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Grant. Obviously this is sub-optimum and we should be looking at a few |
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more things than just Grant in order to preserve Grant. |
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> |
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> > - read failure. You did not see anything that happened prior as it |
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> > was silent. |
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> |
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> If a read successfully returns correct data, how is it "silent"? |
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I never used those words and never said "successfully returns correct |
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data". At best I said something equivalent to "If a read returns". |
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The point I'm trying hard to make is that all our fancy hardware merely |
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gives an *apparency* of reliable results that are totally right or |
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totally wrong. It looks that way because the IT industry spent much |
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time creating wrappers and APIs to give that effect. Under the covers |
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where the actual storage happens it is not like that, and errors can |
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happen. They are rare. |
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Lucky for us, these days we have precision machinery and clever |
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mathematics that reduce the problem vastly. I know in my own case the |
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electronics offer a reliability that far exceeds what I need so I can |
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afford to ignore rare problems. Other people have different needs. |
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-- |
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Alan McKinnon |
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alan.mckinnon@×××××.com |
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