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On 2013-01-08, Alan McKinnon <alan.mckinnon@×××××.com> wrote: |
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> On Tue, 8 Jan 2013 22:15:15 +0000 (UTC) |
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> Grant Edwards <grant.b.edwards@×××××.com> wrote: |
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> |
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>> IMO, having backup data _is_ very valuable, but regularly reading |
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>> files and comparing them to backup copies isn't a useful way to detect |
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>> failing media. |
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> |
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> He doesn't suggest you compare the live data to a backup. He suggests |
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> you compare the current checksum to the last known (presumed or |
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> verified as good) checksum, |
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My point is that comparing the read data with <whatever> is a waste of |
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time if you're worried about detecting media failure. In my |
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expierence, you don't _get_ erroneous data from failing media. You |
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get seek/read failures. |
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> and if they are different then deal with it. "deal with it" likely |
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> involves a restore after some kind of verify process. |
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> |
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> I agree that comparing current data with a backup is pretty pointless - |
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> you don't know which is the bad one if they differ. |
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> |
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> ZFS is designed to deal with this problem by checksumming fs blocks |
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> continually; it does this at the filesystem level, not at the disk |
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> firmware level. |
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I don't understand. If you're worried about media failure, what good |
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does checksumming at the file level do when failing media produces |
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seek/read errors rather than erroneous data? When the media fails, |
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there is no data to checksum. |
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-- |
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Grant |
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