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writing garbage when the voltage drops

writing garbage when the voltage drops

Posted Jul 18, 2006 15:40 UTC (Tue) by giraffedata (subscriber, #1954)
In reply to: writing garbage when the voltage drops by giraffedata
Parent article: Crash-only software: More than meets the eye

Now that I think about it, the atomic write in the case of power failure isn't all that useful, because if the sector doesn't get completely written, it can't be read back. The CRC in the trailer won't have been written. That means you can achieve the same thing by writing two copies of the critical sector: On readback, if you can't read the first copy, you just use the second copy, which is the complete old version.

You'd probably want that redundancy anyway, because it's probably a really important sector and write failures happen even without power failures.

For the benefit of those who are wondering why people think atomic sector writes at power failure are important: Some systems deal with the possibility of system failure in the middle of a complex disk update as follows: Keep the original data intact and write a whole second, updated copy. (Use copy-on-write if you have to for practicality). A single sector points to current copy. When you have a complete updated copy, update the pointer sector to point to the updated copy. Then delete the original copy. Any kind of failure before you update the pointer sector just means the complex update never happened. But if the update of the pointer sector itself gets interrupted, then you've got neither the original nor the updated copy.


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