VDMFEC(1)						      General Commands Manual							 VDMFEC(1)

NAME

       vdmfec - Block ECC for files

SYNOPSIS

       vdmfec [-v] [-d] [-b blocksize] [-n N] [-k K] [file]
       vdm_encode [-v] [-b blocksize] [-n N] [-k K] [file]
       vdm_decode [-v] [-b blocksize] [-n N] [-k K] [file]

DESCRIPTION

       vdmfec reads an input stream and adds error correction blocks so that large consecutive sections of the output stream may be corrupted, and
       the data recovered.  For example, diskettes typically lose whole sectors at once, or related groups of  sectors,  or  even  entire  tracks.
       Data written to a diskette with this program may be recovered even with many read errors.

       The  algorithm  used is a Forward Error Correction (FEC) code based on Vandermonde (VDM) matrices in GF(2^8) due to Luigi Rizzo.  Given the
       FEC parameters K and N, with N greater than K, N blocks are written for every K input blocks in such a way that any K blocks are sufficient
       to reconstruct the data.  That is, up to N - K blocks out of every group of N blocks may be lost without loss of data.

       The  amount  of overhead in the output stream is easily adjustable by varying K.  N and blocksize control the total amount of data written.
       Depending on the types of errors you expect, different settings may be more or less useful.  For example, you may not expect to have two or
       three  bad  sectors  on	every track (if you do it's time to replace the diskette!), but you might expect three bad sectors on two or three
       contiguous tracks (diskette errors tend to cluster).

OPTIONS

       -v     Print informative messages about the encoding or decoding process to stderr, including the expansion factor.  Use -vv to get  infor-
	      mation about each block being read or written.

       -d     Decode the input rather than encoding it.  Using this option is equivalent to invoking the program as vdm_decode.

       -b blocksize
	      Set  the	FEC  block  size to blocksize bytes.  The blocksize may have a 'k' or 'K' appended, in which case the block size is set to
	      blocksize * 1024 bytes.  This should usually be a multiple of the output media block or sector size (e.g., 512,  1024,  18K,  etc.),
	      and must be at least 26 bytes in version 1.  Default 1024.

       -n N -k K
	      Set  the FEC N and K parameters.	N must be greater than 2 and less than or equal to 256.  K must be greater than 0 and less than N.
	      Default N=18, K=14.

       Note that the N, K, and blocksize parameters are NOT written to the output!  You must specify the same parameters when you run the decoder.
       (Actually, the decoder is capable of explicitly detecting an invalid K value, but incorrect blocksize or N values will result in bad blocks
       and decode failure.)

       The decoder is capable of reading from non-seekable media such as pipes, however, buffer underruns are not  detected  and  will	result	in
       failure.   Also,  when reading from a pipe the entire file must be read.  Reading from a seek-able stream can be faster because only K good
       blocks out of N need to be read.

       The encoder stores chunk and block ids which are used to detect out of order blocks, but those errors (which can sometimes  happen  due	to
       filesystem  corruption)	are  not  corrected.  The only type of error that can be corrected is in place corruption of data.  In particular,
       missing blocks (as from lost packets) result in decode failure, even when K good blocks are available.  This program is primarily  intended
       to  recover  data lost due to read errors on fixed media such as diskettes; several other programs and libraries are available that use FEC
       to handle the other (typically network packet) errors.

EXAMPLES

       Consider a diskette with 80 tracks, 2 heads, and 18 512-byte sectors per track (a standard 1.44MB diskette).

	      bzip2 < file | vdmfec > /dev/fd0

       uses the default parameters of -b1k -n18 -k14, stores up to ~1.1MB of compressed input data, written one (double-sided) track  (18K)  at  a
       time, and allows 4 bad clusters (8 sectors) per track.

	      vdmfec -d /dev/fd0 | bunzip2 > file

       recovers the original file.

	      vdm_encode -b18k -n80 -k70 file.img > /dev/fd0

       writes the entire diskette, stores up to ~1.23MB of data, and allows multiple errors on 10 different double-sided tracks.

	      vdm_decode -b18k -n80 -k70 /dev/fd0 > file.img

       recovers  the  data  and reads no less than 70 tracks.  Note that vdm_decode exits with a non-zero status on failure, so that constructions
       such as

	      vdm_decode /dev/fd0 > moo && tar -xvf moo

       are possible.

	      dd if=/dev/tape ibs=1024 conv=noerror,sync | 
	      vdm_decode > file

       This might be necessary to recover data from some non-seekable media.  You can of course simply write the encoded output to a normal  file,
       as in

	      tar -cf - dir | gzip | vdm_encode -v > dir.tgz.vdm

REPORTING BUGS

       Report bugs to Tom Holroyd <tomh@po.crl.go.jp>.

SEE ALSO

       fec(3)

vdmfec								  September 2001							 VDMFEC(1)