[mtdewcmu]

It seems like it might be better to put the metadata at the end. That would make it easier to truncate to a certain amount of entropy. Also, it would make it possible to trim off the metadata without having to understand the format to figure out the variable length.

[panic]

What are you going to do with the hash after you trim off the metadata if you don't understand the format?

[mtdewcmu]

I've dealt with variable-length headers on (binary) things before, and it creates annoyances. It screws up the memory alignment, for instance.

[IanCal]

I have two minor, but useful, uses:

One is that the prefix is probably unique for small sets. So it's both easy to read and parse and then use just a prefix for doing something like stopping a docker container.

Second is that a short prefix on a larger set of data is probably not unique but at a rate I can predict. I can take a random, and repeatable, sample of data by selecting everything with the same prefix. This gives me a fast way of taking a random sample from a database (or file or anything else).

[mtdewcmu]

Good question. There are instances where it allows you to write cleaner (and faster) code if you can identify the parts without necessarily having to parse the metadata.