Purpose of block reserves

[Arne Jansen <sensille@gmx.net>]

Hi,

while reading btrfs source code, I try to make sense of the various uses
of block reserves. The working theory is as follows:

1. Every operation has to reserve upfront every single byte it needs to
   complete its operation fully.
2. If an operation cannot determine how much space it will need, it has
   to be able to cope with running out of space. Normally it does it by
   inserting an orphan item, doing its work in multiple transactions and
   removing the orphan item. The commits in between normally free up enough
   space to continue the operation.
3. All other enospc situations are errors in program logic and should result
   in BUG_ON.

It would be great of someone with a deeper knowledge could correct, expand
or just confirm this.

Thanks,
Arne