An olpcfs experience report

[Joshua N Pritikin]

On Fri, Apr 25, 2008 at 12:12:28PM -0400, C. Scott Ananian wrote:
> In the current olpcfs1 implementation, all metadata is stored in
> Berkeley DB; the actual file contents are stored in a simple
> content-addressable store.

You say "content-addressable store," but what does that mean 
implementation-wise? Does that mean that OLPCFS hashes the content of 
the file into a metadata tag when the file is closed?

In particular, I am wondering what happens when I save a movie made with 
Record (i.e. save a big file). How much additional overhead does OLPCFS 
add?

I presume that the file is not copied an additional time. However, does 
OLPCFS need to read the file again to calculate the hash? If it does, 
can it (eventually in version 2) do this in the background?

> If you use BDB correctly, it seems to me to be pretty solid and mature 
> -- a conclusion which concurs with the long list of BDB users.

Sure, but BDB would be even more trustworthy if I could 'rm -rf' the 
database and regenerate it from scratch without losing data. If metadata 
was stored in POSIX xattrs then it would seem like your design permits 
this mode of operation.

I have seen enough instances of "foolproof" databases that I think we 
should minimize reliance on them.