Hi all,
In order to keep IO request sequence in one pg, osd use pglock to guarantee the sequence. Here in Crimson, it is lockless, so we use future/promise to do the same work.
We can design Each PG has its own IO request queue in seastar-crimson shard. And each PG has one member seastar::promise<> pg_ready;
When need pglock.lock(), we use the following logic to instead:
return pg_ready.get_future() //after satisfy the pg_ready promise later then the future will be fulfilled here.
.then([this] {
Pg_ready = seastar::promise<>{}; // set promise pg_ready no future.
Dequeue io from pg's request queue and do osd following process.
});
When need pglock.unlock(), we use the following logic to instead:
then_wrapped([this] (auto fut) {
fut.forward_to(std::move(pg_ready)); // satisfy the pg_ready promise
});
So the next IO request in the PG queue will not be dequeued until the pg_ready promise is satisfied after the prior request has already been processed in OSD.
Do you think it is workable?
Thanks!
-Chunmei
