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Error-Proofing Data With Reed-Solomon Codes
Posted by
kdawson
on Sun Aug 03, 2008 11:39 PM
from the trust-but-verify dept.
from the trust-but-verify dept.
ttsiod recommends a blog entry in which he details steps to apply Reed-Solomon codes to harden data against errors in storage media. Quoting: "The way storage quality has been nose-diving in the last years, you'll inevitably end up losing data because of bad sectors. Backing up, using RAID and version control repositories are some of the methods used to cope; here's another that can help prevent data loss in the face of bad sectors: Hardening your files with Reed-Solomon codes. It is a software-only method, and it has saved me from a lot of grief..."
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Been doing this from the past 3 decades (Score:2, Interesting)
slow news day anyone?
Re:Been doing this from the past 3 decades (Score:5, Funny)
"Awk! Parroty Error! Parroty Error! Pieces of Seven, Pieces of Seven"
(*BOOM*) never did like that bird.
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It can make files a bit hard to read, though (Score:4, Funny)
Re:It can make files a bit hard to read, though (Score:5, Insightful)
It really depends where you store the FEC, some techniques store the fec separately others concatenate and others interleave the FEC. Each method has its own advantages and disadvantages.
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Re:It can make files a bit hard to read, though (Score:5, Funny)
I for one welcome our Perl Overlords
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Drives already do this (Score:4, Informative)
... at least CDROMs employ RS codes.
Re:Drives already do this (Score:4, Interesting)
My understanding is that it is possible to drill a few holes no larger than 2mm in diameter equally spread over the surface of an "audio cd" and with the help of h/w RS erasure decoding, channel interleaving and channel prediction (eg:probabilistically reconstruct missing right channel from known left channel) one can produce a near perfect reconstruction - that's what usually happens to overcome scratches and other kinds of simple surface defects.
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Re:Drives already do this (Score:5, Informative)
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Re:Drives already do this (Score:4, Informative)
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Re:Drives already do this (Score:5, Informative)
Radial scratches go from center to edge, azimuthal scratches go around the center.
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Re: (Score:3, Funny)
You haven't read too many theses. There are holes all over the place...
Re:Drives already do this (Score:5, Informative)
A CD-ROM sector contains 2352 bytes, divided into 98 24-byte frames. The CD-ROM is, in essence, a data disk, which cannot rely on error concealment, and therefore requires a higher reliability of the retrieved data. In order to achieve improved error correction and detection, a CD-ROM has a third layer of Reed-Solomon error correction.[1] A Mode-1 CD-ROM, which has the full three layers of error correction data, contains a net 2048 bytes of the available 2352 per sector. In a Mode-2 CD-ROM, which is mostly used for video files, there are 2336 user-available bytes per sector. The net byte rate of a Mode-1 CD-ROM, based on comparison to CDDA audio standards, is 44.1k/s×4B×2048/2352 = 153.6 kB/s. The playing time is 74 minutes, or 4440 seconds, so that the net capacity of a Mode-1 CD-ROM is 682 MB.
I'd say that's a yes.
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Re:Drives already do this (Score:5, Interesting)
My biggest failed prediction in the world of computers was the CD-ROM.
I was an audio CD early adopter and I knew from articles I read that audio CD's often had a certain defect rate. The defect rate was usually such that you would never hear it. One artist even published all the defects in the liner notes.
Based upon this, I presumed that you would never get the defect rate to zero and that no one would trust a data medium with anything less than perfection - and thus predicted the CD-ROM would never catch on.
They don't have to get the rate to zero. Just close enough to zero for the RS to function.
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Re:Drives already do this (Score:5, Informative)
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Re:Drives already do this (Score:5, Interesting)
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Re:Drives already do this (Score:5, Funny)
Dude, put the bong down.
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Re:Drives already do this (Score:4, Funny)
Other valid replies would have been "I'll have what he's having" or "Pass it around, please".
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Re: (Score:3, Interesting)
Re:Drives already do this (Score:5, Interesting)
I loved my DAT (for audio) portable recorder, it employed Double-Reed-Solomon error correction, you would have to do some serious hammering to the side of the recorder to get the tape to "skip" in a way the error correction could not correct it and you'd hear it drop out, running and recording was NOT out of the question though.
Now what do the consumers have for recorders - cr*ppy, cheap, nasty, low bitrate, overcompressed MP3 recorders. The recording industry killed off an excellent (but expensive) format to palm off rubbish compressed audio to the masses. (Proper PCM recorders are no different in price to the DAT decks).
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Re: (Score:3, Insightful)
Thing is, the "overcompressed" MP3 recorders are good enough. Most people use them to record lecture notes, or a meeting, or just talking to themselves. Those are about the only reasons to really need a portable recorder, and for those uses, mp3 is very good. Just because it's low bitrate doesn't mean it's bad, and just because your DAT recorder had higher quality doesn't mean it's more fit for the purposes it would be used for. Seriously... running and recording? Why would you ever want to do that?
Yes, RS should be a file system service. (Score:4, Insightful)
I agree. I'm willing to have a small loss in speed and a small increase in price to have better data integrity.
There is already data integrity technology embedded in hard drives, and I support making it more robust.
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Re:Yes, RS should be a file system service. (Score:5, Funny)
Those who sacrifice speed for data integrity deserve neither - BF
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ZFS? (Score:3, Interesting)
Uh, is this not one of the main features of the ZFS file system? It does a checksum on every block written and will reconstruct the data if an error is found? (assuming you are using either raid-z or mirroring. Otherwise it will just tell you that you had an error).
Re:ZFS? (Score:5, Informative)
checksums really only help in detecting errors. Once you've found errors, if you have an exact redundancy somewhere else you can repair the errors. What reed-solomon codes do is provide the error detecting ability but also the error correcting ability whilst at the same time reducing the amount of redundancy required to a near theoretical minimum.
btw checksums have limits on how many errors they can detect within lets say a file or other kind of block of data. A simple rule of thumb (though not exact) is that 16 and 32 bit checksums can detect upto 16,32 bit errors respectively anymore and the chance of not detecting every bit error goes up, it could even result in not finding any errors at all.
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Re:ZFS? (Score:4, Informative)
I have been a ZFS user for a while and know a lot of its internals. Let me comment on what you said.
Not in ZFS. When the checksum reveals silent data corruption, ZFS attempts to self-heal itself by rewriting the sector with a known good copy. Self-healing is possible if you are using mirroring, raidz (single parity), raidz2 (dual parity), or even a single disk (provided the copies=2 filesystem attribute is set). The self-healing algorithm in the raidz and raidz2 cases is actually interesting as it is based on combinatorial reconstruction: ZFS makes a series of guesses as to which drive(s) returned bad data, it reconstructs the data block from the other drives, and then validates whether this guess was correct or not by verifying the checksum.
All the ZFS checksumming algorithms (fletcher2, fletcher4, SHA-256) generate 256-bit checksums. The default is fletcher2 and offers very good error detection (even errors affecting more than 256 bits of data) assuming unintentional data corruption (the fletcher family are not a cryptographic hash algorithms, it is actually possible to intentionally find collisions). SHA-256 is collision-resistant therefore it will in practice detect all data corruptions. It would be computationally infeasible to come up with a corrupted data block that still matches the SHA-256 checksum.
A good intro to the ZFS capabilities are these slides [opensolaris.org]
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Re: (Score:3, Informative)
Mirroring is RAID-1, not 0.
Re: (Score:3, Interesting)
From what I've read and heard, ZFS is designed to pretty much be the last filesystem we'll ever need. I'm pretty sure they've considered hash collisions with regards to data integrity.
Also consider that you probably won't need to reconstruct the entire sector, but only a few bits from it. If there was some sort of insane scenario where you had to reconstruct a complete 1GB block from a single MD5 hash... (ie, "here's an MD5 hash. Give me a sequence of 1073741824 bytes to make it") well it's technically p
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Re:As I understand it... (Score:4, Insightful)
You're asking the wrong question.
The right question is: Given a 1Gb file, how much "mutation" do you have to do to it to produce a file with the same hash? And the answer to that is: Enough to make the data unrecoverable no matter what you do.
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Interesting (Score:2)
I've been burned by scratched DVD+Rs too many times. I'd be interested if there were a way to do this kind of thing in Windows..
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The WinRAR archiver has an optional recovery record which protects against bad blocks.
When you create an archive just specify the amount of protection you require (in practice 3% has served me well).
Re: (Score:3, Informative)
The cross platform program dvdisaster [dvdisaster.net] will add extra information to your DVD as an error correcting code. Alternatively, you can make a parity file for an already-existing DVD and save it somewhere else.
It actually has a GUI too, so it must be user friendly.
Harden Files (Score:3, Funny)
When he said "harden files", I thought he was going into a long soliloquy on all the porn on his computer, so I went to the next story.
Never underestimate the redunancy... (Score:3, Interesting)
Look, if it's secret, one copy is too many. For everything else, gmail it to five separate recipients. It's not like Google has ever lost any of the millions of emails I've received to date. (This is not a complaint -- they don't show me the spam unless I ask for it).
And if they ever did lose an email, well, to paraphrase an old Doritos commercial, "They'll make more."
Seriously, personally I view the the persistence of data as a problem. It's harder to let go of than it is to keep.
Speed? (Score:4, Interesting)
My question is of speed; this seems a promising addition to anyone's back up routine. However, most folks I know have 100s of gigs of data to back up. While differentials could be involved, right now tar'ing to tape works fast enough taht the backup is done before the first staff shows up for work.
I assume we're beating the hell out of the processor here; so I'm wondering how painful is this in terms of speed?
Re: (Score:3, Informative)
Well since my $100 Radio Shack CD [wikipedia.org] player I bought in 1990 could do it in real-time I'm guessing that the requirements are pretty low. In fact a lot of hardware already uses it.
If you read the rest of the page you find out it's very ingenious and efficient at doing what it does.
While it's certainly not new (it's from 1960) or unused (hell, my phone uses it to read QR codes) I'm sure its something that has been under the radar of a lot of Slashdot readers, so I'll avoid making a "slow news day" snark.
Re:Speed? (Score:5, Interesting)
The speed of encoding and decoding directly relates to the type of RS and the amount of FEC required. Generally speaking erasure style RS can go as low as O(nlogn) (essentially inverting and solving for a vandermonde or Cauchy style matrix) A more general code that can correct errors (the difference between an error and an erasure is that in the latter you know the location of the error but not its magnitude) may require a more complex process, something like Syndrome-Berlekamp Massey-Forney which is about O(n^2).
It is possible to buy specialised h/w (or even GPUs) to perform the encoding steps (getting roughly 100+MB/s) and most software encoders can do about 50-60+Mb/ for RS(255,223) - YMMV
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Version control != backups (Score:3, Insightful)
Re: (Score:3, Insightful)
Well, you shouldn't commit until you believe you have it in a state where the changes are usable (i.e. don't break the tree), but beyond that, I'd rather see more commits of smaller amounts of code than giant commits that change ten thousand things. If you end up having to back out a change, it's much easier if you can easily isolate that change to a single commit. My rule is commit early, commit often. I'm not the only one, either:
http://blog.red-bean.com/sussman/?p=96 [red-bean.com]
Re:Version control != backups (Score:4, Insightful)
The best solution is for developers to use their own private branches. Then they can commit as much as they want, and integrate into the main branch when they're ready. Unfortunately subversion has crappy support for integration (even with version 1.5 AFAICT) compared to something like perforce.
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Or you could just use PAR... (Score:3, Interesting)
TFA introduces some new ".shielded" file format. But do we need yet another file format when PAR (Parchive) [wikipedia.org] has been doing the same job for years now? The PAR2 format is standardized and well-supported cross-platform, and might just have a future even IF you believe that Usenet is dying [slashdot.org]...
I always thought it would be cool to have a script that:
With a system like this, you wouldn't have to worry about throwing away old backups for fear that some random bit error might have crept into your newer backups. Also, if you back up the PAR2 files together with your data, as your backup media gradually degrades with time, you could rescue the data and move it to new media before it was too late.
Of course, at the filesystem level there is always error correction, but having experienced the occasional bit error, I'd like the extra security that having a PAR2 file around would provide. Also, filesystem-level error correction tends to happen silently and not give you any warning until it fails and your data is gone. So a user-level, user-adjustable redundancy feature that's portable across filesystems and uses a standard file format like PAR would be really useful.
This is not the same thing as PAR ... (Score:5, Informative)
The difference is that TFA interleaves the data so it is robust against sector errors. A bad sector contains bytes from many different data blocks so each data block only loses one byte which is easy to recover from. If you use PAR and encounter a bad sector, you're SOL.
PAR was designed to solve a different problem and it solves that different problem very well but it wasn't designed to solve the problem that is addressed by TFA. Use PAR to protect against "the occasional bit error" as you suggest, but use the scheme given in TFA to protect against bad sectors.
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what about quickpar and dvdisaster? (Score:5, Informative)
quickpar [quickpar.org.uk] especially has been in use on usenet/newsgroups for years....o yea...forgot....they are trying to kill it.
anyways...there's also dvdisaster [dvdisaster.net] which now has several ways of "hardening".
one of them seems to catch my attention: adds error correction data to a CD/DVD (via a disc image/iso)
Datarecovery "data". (Score:5, Insightful)
Working for a datarecovery company, I know that about half the cases where data is lost the whole drive "disappears". So, bad sectors? You can solve that problem with reed solomon! Fine! But that doesn't replace the need for backups to help you recover from: accidental removal, fire, theft and total disk failure (and probably a few other things I can't come up with right now)... .
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I'm still waiting for the 5 year warranty to expire on my hard drives.
huh??? (Score:3, Insightful)
I have been around this industry quite a while, and I call bullshit on that.
Re: (Score:3, Informative)
Your comment is incorrect, RS codes are a subset of BCH codes. In fact BCH codes are a general definition of a class of algebraic codes nothing more. your comment about one being better than the other for a specific purpose is wrong.
Think of BCH codes as "vehicle" and RS codes as "The Bugatti Veyron" that is the relationship.
Re:You are all dumb as there is only one way. (Score:4, Interesting)
Reed-Solomon is ancient compared to par2.
No, you're dumb. Par2 IS Reed-Solomon. Silly me to expect an AC to fact-check the most trivial subjects of a post.
The procedure explained in TFA is basically adapting a different tool to behave more or less like single-file par2. That makes it redundant (in the /. sense, not the data-recovery sense).
There is one thing I would love to see, and that's local disk checksumming. That's right, take a 500gb disk, chop it into slices and do RAID-5 on them as if they were individual spindles. It's been years since I've had a hard drive actually die on me, but I've seen bit-errors more often than I'd like. Having self-checking built into the filesystem (or low-level disk access) would help ensure 100% data integrity, and you could still do RAID-1 on top of it for safety.
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Re: (Score:3, Informative)
Unfortunately this software looks like it is closed source and windows only. A program to apply error correcting codes to your archived files is only useful if you still have a platform to run it on. Hopefully 15 years from now when you go to recover your files y