this doesn't sound right... but maybe thats because I sort of assume any "sufficiently smart file system" has sort of COW (copy on write) like append semantics

this doesn't sound right... but maybe thats because I sort of assume any "sufficiently smart file system" has sort of COW (copy on write) like append semantics plus some sort of amortized compaction similar to a reasonable point in the GC design space of copying vs mark-sweep. (realistically most File systems don't do something like that, but its what I imagine "should be" the case)