Following up from my previous post.

I used the API at https://archive.org/developers/changes.html to enumerate all the item names in the archive. Currently there are over 256 million item names. However I went through a sample of them and noted the following:

• Many do not have the .torrent available because some of the files are locked due to copyright concerns, like their music collection. Ex: https://archive.org/details/lp_le-sonate-per-pianoforte-vol-1_carl-maria-von-weber-dino-ciani_0
• A lot of items have been removed from public access completely, and possibly deleted even on their storage backend. Ex: https://archive.org/details/0-5-1-0-hernan-hernandez

There are many, many items from the archive which have been removed. Much higher than I expected. If you have critical data, of course Internet Archive should never be your only backup.

I don't know the distribution of metadata and .torrent file sizes since i have not tried downloading them yet. It looks like it would require a lot of storage if there are many files or the content is huge (if only 50% of the items remain and the average .torrent + metadata is 20KB it would be over 2.5 TB to store). But on the other hand, the archive has a lot of random one off uploads that are not very big, so some metadata is 800 bytes and the torrent 3KB in those cases (only 640 GB to store if combined is 5 KB).