a very annoying property: if the source media is slow (like a slow
network), the target file will be locked for the duration of the
entire max 8MB write and cause processes attempting to e.g. stat()
it to "tstile" (for several minutes in the worst case). Revisit
this if/when vnode locking gets a little smarter.
chs ok (although he would rather see vnode locking improvements,
but that's a slightly larger project)
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