[lxpz]

> Can you do conditional writes?

If by that you mean something like compare-and-swap or test-and-set, then no, that would require consensus number > 1 [0]

> Can old versions of a register be compacted to reduce metadata space?

Yes, when using last-write-wins registers, old versions are deleted as soon as they are overseded by a newer version. Almost all metadata stored in Garage uses this mechanism.

> How does garbage collecting dead blobs work?

Nodes have a shared table of block references, that identifies for each data block the set of objects (or rather, versions of objects) that use this block. Block references have a "deleted" flag that is set to true when the version in question is deleted from the store. Once set to true, it cannot go back, but a new version can always be created that references the same block. Nodes keep a local count of the number of non-deleted block references for each block, so that when it reaches zero, they queue up the block for deletion (after a small delay). There is also a metadata GC system so that block references with the deleted flag are dropped from the table after a certain time.

[0] https://en.wikipedia.org/wiki/Consensus_(computer_science)#C...