We no longer wait on the semaphore and instead over-subscribe it, so
there's not reason to pass a timeout.
Signed-off-by: Duarte Nunes <duarte@scylladb.com>
We arrive at an overloaded state when we fail to acquire semaphore
units in the base replica. This can mean clients are working in
interactive mode, we fail to throttle them and consequently should
start shedding load. We want to avoid impacting base table
availability by running out of memory, so we could offload the memory
queue to disk by writing the view updates as hints without attempting
to send them. However, the disk is also a limited resource and in
extreme cases we won’t be able to write hints. A tension exists
between forgetting the view updates, thereby opening up a window for
inconsistencies between base and view, or failing the base replica
write. The latter can fail the whole user write, or if the
coordinator was able to achieve CL, can instead cause inconsistencies
between base tables (we wouldn't want to store a hint, because if the
base replica is still overloaded, we would redo the whole dance).
Between the devil and the deep blue sea, we chose to forget view
updates. As a further simplification, we don't even write hints,
assuming that if clients can’t be throttled (as we'll attempt to do in
future patches), it will only be a matter of time before view updates
can’t be offloaded. We also start acquiring the semaphore units using
consume(), which is non-blocking, but allows for underflow of the
available semaphore units. This is okay, and we expect not to underflow
by much, as we stop generating new view updates.
Refs #2538
Signed-off-by: Duarte Nunes <duarte@scylladb.com>
When generating view updates from a staging sstable, this sstable
should not be used in the process. Hence, a reader that skips a single
sstable is added.
After materialized view updates are generated, the sstable
should be moved from staging/ to a regular directory.
It's expected to be called from seastar::async thread context.
