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Am 08.01.2013 18:41, schrieb Pandu Poluan: |
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> |
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> On Jan 8, 2013 11:20 PM, "Florian Philipp" <lists@×××××××××××.net |
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> <mailto:lists@×××××××××××.net>> wrote: |
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>> |
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> |
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> -- snip -- |
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> |
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[...] |
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>> |
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>> When you have completely static content, md5sum, rsync and friends are |
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>> sufficient. But if you have content that changes from time to time, the |
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>> number of false-positives would be too high. In this case, I think you |
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>> could easily distinguish by comparing both file content and time stamps. |
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>> |
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[...] |
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> |
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> IMO, we're all barking up the wrong tree here... |
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> |
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> Before a file's content can change without user involvement, bit rot |
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> must first get through the checksum (CRC?) of the hard disk itself. |
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> There will be no 'gradual degradation of data', just 'catastrophic data |
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> loss'. |
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> |
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Unfortunately, that's only partly true. Latent disk errors are a well |
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researched topic [1-3]. CRCs are not perfectly reliable. The trick is to |
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detect and correct errors while you still have valid backups or other |
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types of redundancy. |
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The only way to do this is regular scrubbing. That's why professional |
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archival solutions offer some kind of self-healing which is usually just |
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the same as what I proposed (plus whatever on-access integrity checks |
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the platform supports) [4]. |
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> I would rather focus my efforts on ensuring that my backups are always |
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> restorable, at least until the most recent time of archival. |
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> |
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That's the point: |
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a) You have to detect when you have to restore from backup. |
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b) You have to verify that the backup itself is still valid. |
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c) You have to avoid situations where undetected errors creep into the |
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backup. |
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I'm not talking about a purely theoretical possibility. I have |
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experienced just that: Some data that I have kept lying around for years |
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was corrupted. |
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|
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[1] Schwarz et.al: Disk Scrubbing in Large, Archival Storage Systems |
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http://www.cse.scu.edu/~tschwarz/Papers/mascots04.pdf |
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|
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[2] Baker et.al: A fresh look at the reliability of long-term digital |
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storage |
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http://arxiv.org/pdf/cs/0508130 |
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|
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[3] Bairavasundaram et.al: An Analysis of Latent Sector Errors in Disk |
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Drives |
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http://bnrg.eecs.berkeley.edu/~randy/Courses/CS294.F07/11.1.pdf |
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[4] |
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http://uk.emc.com/collateral/analyst-reports/kci-evaluation-of-emc-centera.pdf |
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Regards, |
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Florian Philipp |
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