On Mon, Feb 08, 2016 at 12:34:04 CET, Michael Kjörling wrote: > On 8 Feb 2016 01:25 +0100, from sven@xxxxxxxxxxxxxxxxxxxxx (Sven Eschenberg): > > I always > > wondered how a HDD exactly behaves when power fails, while a sector > > is in transit. My best hope is, that the CRC at the end of the > > sector does not match and an error is returned on the next read? > > That's the theory; if a sector write is interrupted half way through > (regardless of the reason), then the FEC data doesn't match the sector > payload data. In this case, the difference is very likely large enough > that the error cannot be corrected using the FEC data, so you get a > read error back instead. > > _Unfortunately_, theory and practice don't always agree. I think it > was Google that did a study on storage errors not all that long ago, > and one conclusion was that silent read errors (where you do get data > back from the drive, but that data is not the same as was originally > written), while rare, happens with a high enough probability to > warrant consideration in large storage systems. I think I read that paper (if so, it was pretty bad) and if I remember correctly, they did not diagnose what the issues were, just that they had bad data at the end in main memory. >From my experience shoveling a few hundred TBs of research data around when 200GB disks where standard, the only undetected errors I ever found were due to memory corruption due to a weak RAM bit in one server that did not have ECC memory. Those amounted to 3 errors in 30TBs of recorded data. I never had undetected read errors from disk (and since all data was bzip2 compressed, errors would have been found), so I tend to view these as not a disk problem, but likely happening someplace after the data leaves the disk. Regards, Arno -- Arno Wagner, Dr. sc. techn., Dipl. Inform., Email: arno@xxxxxxxxxxx GnuPG: ID: CB5D9718 FP: 12D6 C03B 1B30 33BB 13CF B774 E35C 5FA1 CB5D 9718 ---- A good decision is based on knowledge and not on numbers. -- Plato If it's in the news, don't worry about it. The very definition of "news" is "something that hardly ever happens." -- Bruce Schneier _______________________________________________ dm-crypt mailing list dm-crypt@xxxxxxxx http://www.saout.de/mailman/listinfo/dm-crypt
