>  1. Is the Marshal module that much unstable? I have the feeling that
>     marhsaled values are compatible between OCaml releases. True or
>     false?

True in practice: the last incompatible change was done in 1996...
Still, the Marshal module is not intended for long-term storage of
data that you cannot easily reconstruct from other sources.  In
particular, the data format is compressed enough that it is not
possible to salvage data using, say, a text editor.  At the very
least, it is prudent to build into your software functions to dump and
restore marshaled data to/from a simpler, textual format.  (That's
what I did for SpamOracle's database, which is a marshaled hash table.)

>  2. Any advice on implementing persistent storage? I know about Persil
>     library[3] but I don't see much advantage of using it (does it
>     implement its own marshaling, more stable than Marshal?).

There are two independent questions:

1- How to encode your data into character strings?
2- How to store these strings in persistent storage?

For 2-, you have many options, ranging from flat files (the
traditional Unix way) to full-fledged databases.

For 1-, in addition to the Marshal module, you can also use textual
formats: key-value pairs, XML, Lisp S-expressions, etc.  Other
options are XDR or ASN1 encodings.

- Xavier Leroy