b9be892bf9 has made Persist asynchronous which
is very effective in allowing the system to continue processing
blocks/transactions while flushing things to disk. It at the same time is very
dangerous in that if the disk is slow and it takes much time to flush KV set
(more than persisting interval), there might be even bigger new KV set in
MemCachedStore by the time it finishes. Even if the system immediately starts
to flush this new data set it (being bigger) can take more time than the
previous one. And while doing so a new data set will appear in memory,
potentially again bigger than this.
So we can easily end up with the system going out of control, consuming more
and more memory and taking more and more time to persist a single set of
data. To avoid this we need to detect such condition and just wait for Persist
to really finish its job and release the resources.
5fbe838fd4