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disk error bursts
The frequency with which disk sectors become unreadable seems to have
remained relatively stable over the years, though one might hope that it will decrease significantly with the expected move to 4 KB sectors (at least if correction bits per sector increase somewhere nearly commensurately). But it's difficult to find information about how frequently error bursts that result in unreadability span a sector boundary. In particular, I'm interested in whether two successive sectors ever (in a practical and quantifiable sense) become unreadable due to a single error burst, and if so whether such a burst ever affects more than two successive sectors (it would also be interesting to know how frequently bursts span tracks such that they affect sectors on adjacent tracks, though that is of less immediate significance to me). If such large error bursts tend to be the result of damage caused by physical contact with the disk head that is likely to destroy significant portions of the platter that information would be interesting as well, since my main concern involves the incidence of two-successive-sector and more-than-two-successive-sector unreadability caused by 'bit-rot' compared with that of single-sector unreadability. Does anyone know of accessible studies providing such information? Thanks, - bill |
#2
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disk error bursts
In comp.arch Bill Todd wrote:
But it's difficult to find information about how frequently error bursts that result in unreadability span a sector boundary. Well, ancedotally speaking, most of the disk failures I've seen on my systems have involved clusters of often sequentially unreadable blocks. Does anyone know of accessible studies providing such information? I imaginge thse guys would be good people to ask... http://research.google.com/pubs/pub32774.html G. |
#3
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disk error bursts
Bill Todd wrote:
There hasn't been much discussion on this, so, with all the same caveats I mentioned on my previous posts on disk technology, I'll give some information. The frequency with which disk sectors become unreadable seems to have remained relatively stable over the years, Yes. though one might hope that it will decrease significantly with the expected move to 4 KB sectors Probably not - see below. (at least if correction bits per sector increase somewhere nearly commensurately). Not exactly. First of all, is the move to 4K sectors going to happen? One of the last things I did before I retired 10 years ago was to attend a meeting, hosted by IBM Research in San Jose, attended by all the major disk manufacturers, to discuss this issue. I was the "systems guy", as I was the only one who wasn't a disk device engineer type. I talked about the issues relating to the interface (i.e. ATA had no capability to specify a sector length so I talked about alternatives for backwards compatibility) and the people who, for various reasons, didn't use exactly 512 bytes per sector and what would be needed to satisfy them with larger sector sizes. Anyway, since the transition to 4K sectors hasn't happened in ten years, I sort of assumed that the issue was long dead and wasn't going to happen at all, but I have been out of it, so I may very well be wrong. Now, getting back to newly bad sectors. What was causing most of them was a phenomenon called thermal asperities (TA). These are small (a single digit number of bits long) "bumps" in the surface of the media caused by additional particles sputtered on the media. They were not high enough to touch the heads when the heads were flying at normal height. But if the fly height were at its lower margin, the heads could "graze" the top of the asperity. This didn't damage the heads, but the friction could generate enough heat to cause a local erasure of the data. If the data was rewritten, things would be fine again. The asperities were randomly distributed over the disk. To counteract these TAs, the vendors used variants of Reed Solomon ECC codes that could correct up to two bursts of say 10-12 bits per sector and detect up to three such bursts. This allowed correction of up to two TAs per sector. Longer defects were generally caused by a 'scratch" where the head contacted the media for a longer period of time, and these were of arbitrary length. If they were short enough, they could be corrected by the ECC. Of course, if they were too long, it represented a head crash and it really didn't matter what ECC you used. :-( You would expect the size in bits of the TAs to get larger with increasing BPI, but process improvements seemed to just about keep up, so there wasn't much growth in bit length correction capability. So, getting back to longer sectors, I would expect vendors to use ECCs that have the capability to detect and correct more TAs per sector, not longer bursts per say. Of course, if the errors were consecutive, adding more bursts to the span does increase the length of a single error you can correct, but that is secondary. But it's difficult to find information about how frequently error bursts that result in unreadability span a sector boundary. And, even if you did, probably not useful. Remember there is a lot of stuff between sectors on a disk. That is, they are not consecutive bits around the track. Besides the ECC, there are sync fields, gaps, and occasional servo bursts (headers are long gone). So depending on the details, you can't correlate consecutive sector losses with what would happen with longer sectors. In particular, I'm interested in whether two successive sectors ever (in a practical and quantifiable sense) become unreadable due to a single error burst, and if so whether such a burst ever affects more than two successive sectors (it would also be interesting to know how frequently bursts span tracks such that they affect sectors on adjacent tracks, though that is of less immediate significance to me). Just as a matter of interest, disk vendors have scratch detection algorithms for scratches that run radially. Thus, if during formatting, they detect an error on the same sector (modulo effects like record number skew and zoned bit recording) in say tracks 30,31,33 and 34, they will mark the corresponding sector in track 32 as bad in order to prevent a possible error later. But in direct answer to your question, I don't know of anyone who has the statistics that you want. -- - Stephen Fuld (e-mail address disguised to prevent spam) |
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