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Power loss -> no guarantee?

Power loss -> no guarantee?

Posted Sep 17, 2009 11:11 UTC (Thu) by forthy (guest, #1525)
In reply to: Only with a UPS by ncm
Parent article: POSIX v. reality: A position on O_PONIES

This is wrong. Consider a log-structured, checksummed file system like NILFS. It gathers all writes, writes them out in one go, and checksums every chunk it writes. What happens when power is lost during that write? The checksum is wrong. The last update before isn't touched, so the file system will revert to this last update. All is hunky dory, all ponies still there, no data lost except the last update - which is the guarantee of such a file system: You can only depend that those data is on disk where the transaction was completely written to disk. And note: writing one sector to a hard disk takes a few microseconds nowadays, so the drive can detect a power outage and stop writing before it randomly scrambles a sector - it might not complete everything, but leaving a garbled sector is possible to avoid.

On the other argument: In the part of the world where I live (Munich), power outages are far less frequent than crashes. Our file server had some CPU problems two years ago and crashed about once a week. Thanks to the stability of ReiserFS, no data loss occurred during the half year until we found the root cause and replaced the CPUs. Even when not including hardware defects, I definitely have more crashes than power outages. Frequent power outages happen in poor countries with third-world infrastructure.


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