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Re: Known limitation with TokuDB in Read Free Replication & parallel replication ?


jocelyn fournier <jocelyn.fournier@xxxxxxxxx> writes:

> Thanks for the quick answer! I wonder if it would be possible the
> automatically disable the optimistic parallel replication for an
> engine if it does not implement it ?

That would probably be good - though it would be better to just implement
the necessary API, it's a very small change (basically TokuDB just needs to
inform the upper layer of any lock waits that take place inside).

However, looking more at your description, you got a "key not found"
error. Not implementing the thd_report_wait_for() could lead to deadlocks,
but it shouldn't cause key not found. In fact, in optimistic mode, all
errors are treated as "deadlock" errors, the query is rolled back, and
run again, this time not in parallel.

So I'm wondering if there is something else going on. If transactions T1 and
T2 run in parallel, it's possible that they have a row conflict. But if T2
deleted a row expected by T1, I would expect T1 to wait on a row lock held
by T2, not get a duplicate key error. And if T1 has not yet inserted a row
expected by T2, then T2 would be rolled back and retried after T1 has
committed. The first can cause deadlock, but neither case seems to cause
duplicate error.

Maybe TokuDB is doing something special with locks around replication, or
something else goes wrong. I guess TokuDB just hasn't been tested much with
parallel replication.

Does it work ok when running in conservative parallel mode?

 - Kristian.

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