On Mon, 2005-12-12 at 21:47 -0600, Mike McCarty wrote: --snip-- > This is not quite true. Usually, with a disc file or a file transmitted > over a link of some sort, a block of bytes gets corrupted, usually with > a size on the order of 1KB. If an archive is much larger than 1KB (the > usual case, in my experience) then most of the archive may actuallly > be usable. If each file contained in the archive has a separate > redundancy check, then one may be able (in my experience, usually can) > do a partial recovery, and extract some of the information. In one case, > I had backups which got corrupted (due to a virus overwriting sectors > on floppy discs). I had the same file corrupted in different places, > and was able to recover the entire archive (it was a PKZIPped archive) > by doing partial recovery on two different backups of the same archive.
This is a good point. But this is also not a feature specific to RAR, as you point out. Zip archives, among others, can do this as well. --snip-- > Why do you suppose we put cyclic redundancy checks into messages > used in various protocols, like FTP, TCP, IP, MTP, etc? By your > reasoning, ISTM, you would view such cyclic redundancy checks to > be invalid. They should be sent via a separate message. But then > what checks the separate messages? Or in your case, what checks > the separate files? How do you know your separate files containing > the redundancy checks aren't corrupt? Do you have another set of > files checking the check files? And what checks the check check files? > > To put it another way, are you aware that hard discs have redundancy > in them to allow one to recover corrupted sectors? By your reasoning, > this could not be put on the same disc, it would have to be on > another disc. Yes, but CRC checks on packets of data (a few kilobytes at a time) are much different than a checksum on a 10, 20, or 50 MB archive, particularly when speaking in terms of a lower-bandwidth connection. It's easy enough to resend a TCP/IP packet if the CRC check fails. However, we want to AVOID resending a 50 MB file if the checksum doesn't agree with the file. It's much easier to re-download a 1K checksum file than a 50 MB archive. WRT to your statement about hard drives, I believe the feature you're referring to is the ability to ignore bad sectors, not recover data from them. If the drive determines a particular sector to be bad, it will mark it as being bad and write the data elsewhere. If the data has already been written to a sector, and that sector subsequently goes bad I'm not aware of any low-level features to allow the recovery of that sector after the fact. --snip-- > (1) the ability to open and extract files from previously created RAR > format archives, and (2) the exchange of files with people who as a > customary practice use that format for archiving and compression > and prefer to send and receive them in that format. (1) unrar-free would do this as well (2) see below > I can't control my neighbor's dog, let alone my neighbor. If > someone I am doing a contract for requests RAR format, then > that's what he'll get, and I won't argue. I consider putting > food on my table a very practical thing. I completely agree. I'm not fortunate enough to be able to make a living evangelizing free software either. (I work at a 99% MS shop. Used to be 100% until I got there, so I'm making inroads. :) ) However, the original question referred to how to archive a large amount of data and split it into pieces for transferring to someone else. If someone REQUESTS RAR specifically, and I have a business relationship with them, I'll certainly oblige. However, if the format is unspecified, as in this case it was, I would certainly choose to use free software to do the job. -- Alex Malinovich Support Free Software, delete your Windows partition TODAY! Encrypted mail preferred. You can get my public key from any of the pgp.net keyservers. Key ID: A6D24837
signature.asc
Description: This is a digitally signed message part
