> It's not appealing to chop that 1TB into a thousand PAR2 files with extra 20% redundant parity and posting it to alt.binaries.backup.
For a 1 TB archive with 20% redundancy, you're looking at a block size of at least 32 MB in each par2 file (due to the maximum block count of 32767 [1] in the official implementation). Given that the article size limit for many news servers is roughly 1 MB, you're looking at even a single block getting split into 32 article posts. par2 programs typically will generate a series of files where the smallest files contain a single block and the largest files contain 50 or more blocks. The 50 block files will each get split into 1600 articles.
For par2 recovery to work even when articles are missing, you really want the recovery block size to be less than the article size limit, so that even if one or more articles are missing, the par2 archiver program can still read a subset of blocks from the incomplete recovery file and still use them for recovery. That means that the maximum archive size would be roughly 32 GB to keep the block size under the article size limit.
Going beyond that size means that it's less likely that the recovery file would be usable if some of the articles are missing. At 32 GB, if one article is missing from a 3 block recovery file, the software will still be able find 2 blocks in that file. But, if the archive size was 100 GB, then the block size would be a minimum of 3 MB and just missing 3 out of 9 articles that make up a 3 block recovery file would make the recovery file unusable.
For a 1 TB archive with 20% redundancy, you're looking at a block size of at least 32 MB in each par2 file (due to the maximum block count of 32767 [1] in the official implementation). Given that the article size limit for many news servers is roughly 1 MB, you're looking at even a single block getting split into 32 article posts. par2 programs typically will generate a series of files where the smallest files contain a single block and the largest files contain 50 or more blocks. The 50 block files will each get split into 1600 articles.
For par2 recovery to work even when articles are missing, you really want the recovery block size to be less than the article size limit, so that even if one or more articles are missing, the par2 archiver program can still read a subset of blocks from the incomplete recovery file and still use them for recovery. That means that the maximum archive size would be roughly 32 GB to keep the block size under the article size limit.
Going beyond that size means that it's less likely that the recovery file would be usable if some of the articles are missing. At 32 GB, if one article is missing from a 3 block recovery file, the software will still be able find 2 blocks in that file. But, if the archive size was 100 GB, then the block size would be a minimum of 3 MB and just missing 3 out of 9 articles that make up a 3 block recovery file would make the recovery file unusable.
[1] https://en.wikipedia.org/wiki/Parchive#Parity_Volume_Set_Spe...