Il 2014-08-04 20:40 Mikael Abrahamsson ha scritto:
Why do you think that's wrong? 10^-14 is what the vendor guarantees. I have had drives with worse performance (after a couple of months I had several UNC sectors without reading much). Your claim about the article being wrong is the same as saying that the risk reported of getting into a car accident is wrong because you've driven that amount of kilometers but haven't been in an accident yet. This is statistics, marketing and warranty, not guaranteed behavior.
Yes, I understand this. However, the linked article (and many others) state: "If you have a 2TB drive, you write 2TB to it, and then you fully read that, just over 6 times, then you will run into one read error, theoretically speaking."
I read my 500 GB drive over _60_ times, reading 3x more total data than stated above.
I started the entire discussion to know how UREs are calculated, trying to understand if they are expressed as probability ("1 probabily over 10^14 that we can not read a sector) or a statistical record ("we found that 1 on 10^14 is not readable").
If defined as a probability, I am very lucky: if my math is OK, I should have only 0.5% to read about 40 TB of data (my math is: (1-(1/10^14))^(3*(10^14))). If, on the other hand, UREs are defined as statistical evidence (as MTBF), environment and test conditions (eg: duty cycle, read/write distribution, etc) are absolutely critical to understand what this parameter really mean for us.
I'm under impression (and maybe I'm wrong, as usual :)) that UREs mainly depends on incomplete writes and/or unsable sectors. If this is the case, maybe the published URE values are related to the entire HDD warranty. In other word, they should be read as "in normal condition, with typical loads, out HDD will exibit about 1/10^14 unrecoverable error during the entire disk lifespan".
It is reasonable? Or I am horribly wrong? Regards. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@xxxxxxxxxx - info@xxxxxxxxxx GPG public key ID: FF5F32A8 -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
