ATM, the replication logic sends all dry-run requests in parallel,
which might overwhelm the ZFS pool on the sending side.
Since we use rpc/dataconn for dry sends, this also opens one TCP
connection per dry-run request.
Use a sempahore to limit the degree of concurrency where we know it is a
problem ATM.
As indicated by the comments, the cleaner solution would involve some
kind of 'resource exhaustion' error code.
refs #161
refs #164
We assumed that `zfs recv -F FS` would basically replace FS inplace, leaving its children untouched.
That is in fact not the case, it only works if `zfs send -R` is set, which we don't do.
Thus, implement the required functionality manually.
This solves a `zfs recv` error that would occur when a filesystem previously created as placeholder on the receiving side becomes a non-placeholder filesystem (likely due to config change on the sending side):
zfs send pool1/foo@1 | zfs recv -F pool1/bar
cannot receive new filesystem stream:
destination has snapshots (eg. pool1/bar)
must destroy them to overwrite it
A bookmark with a well-known name is used to track which version was
last successfully received by the receiver.
The createtxg that can be retrieved from the bookmark using `zfs get` is
used to set the Replicated attribute of each snap on the sender:
If the snap's CreateTXG > the cursor's, it is not yet replicated,
otherwise it has been.
There is an optional config option to change the behvior to
`CreateTXG >= the cursor's`, and the implementation defaults to that.
The reason: While things work just fine with `CreateTXG > the cursor's`,
ZFS does not provide size estimates in a `zfs send` dry run
(see acd2418).
However, to enable the use case of keeping the snapshot only around for
the replication, the config flag exists.
