[Libguestfs] Thoughts on virt-builder --output-alloc sparse|preallocated

I had a good go at implementing $subject, but it's not as easy as it seems. There was a customer request for this, although I don't think they ended up filing a BZ for it. Anyway, these are my random thoughts in no particular order. It would be nice to use `-oa sparse' etc for compatibility with virt-v2v, but that possibly closes off future options for replacing the horrible OCaml Arg module with GNU getopt_long (-oa == -o + -a). I initially tried to implement `--output-alloc sparse|preallocated' using the Planner. I added an extra planner flag [`Alloc, ("sparse"|"preallocated")]. I added extra transitions such as `Copy_sparse_never ("cp --sparse=never"). The problem is that some output locations don't support sparse output, eg. writing to a block device. If the user did: virt-builder --output-alloc sparse --output /dev/sdX then the planner stage would actually fail because it couldn't find a satisfactory plan. (Even worse because `--output-alloc sparse' is the default!) So then I tried to add `--output-alloc prefer-sparse' as the default. The problem with that is the planner could choose to use a preallocating transition (eg. "cp --sparse=never" or "qemu-img ... -o preallocation=full"), when ideally we'd always want it to choose sparseness if possible. It may be possible to use the planner weights to avoid this, but I'm not convinced the way we handle weights is very robust (in particular, plans with lower total weight but more steps are ignored). Adding some kind of "best effort" goal to the planner might be possible, but our planner is very naive and doesn't support that at the moment. (All current goals must be "must" or "must not".) Anyhow, there you go. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com Fedora Windows cross-compiler. Compile Windows programs, test, and build Windows installers. Over 100 libraries supported. http://fedoraproject.org/wiki/MinGW