mmapFileEnable() - Enables Memory Mapped Files, much faster storage option. However on 32bit JVM this mode could corrupt your DB thanks to 4GB memory addressing limit.You genius!

I think you actually solved the mystery!
Very good job, DeMap!

PS: I've been running Qora on 64-bit windows (64-bit JVM), and I can attest that my Qora client never once encountered any problem (No exception thrown)... I'm pretty sure it's a JVM problem.
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PS 2: If this is the case, what Poloniex needs to do is to compile a new Qora wallet with memory map file always enabled, and switch to 64-bit JVM. I think someone needs to contact Poloniex or Tristen and let them know about the solution.
Indeed, this seems to be a workaround that solves this problem
https://groups.google.com/d/msg/mapdb/9MBNRFbX3g4/otD5fYEQBM0JClosedChannelException happens if one of the threads is interrupted while doing IO. The FileChannel gets closed on interruption and all future IOs will throw CCE. There is workaround to use memory-mapped files (mmapFileEnable()), but they only work on 64bit systems.
So either enable mmap or investigate what is interrupting your threads.
https://github.com/jankotek/MapDB/issues/352#issuecomment-48128816 Some threads are interrupted while reading/writing from FileChannel. As result FC is closed and always throws ClosedByInterruptException. That is documented behavior of FileChannel, and there is not much I can do without sacrificing performance.
So my suggestion is to stop interrupting threads while they are reading/writing.
Other solution is to use memory mapped files (mmapFileEnable()). Perhaps they are not affected by this
https://github.com/jankotek/MapDB/issues/384#issuecomment-68485183 I would recommend to use mmap files as
alternative to prevent this.
Also make sure that no thread is being
interrupted while doing IO. That would close
FileChannel
mmapFileEnable public DBMakerT mmapFileEnable()
Enables Memory Mapped Files, much faster storage option. However on 32bit JVM this mode could corrupt your DB thanks to 4GB memory addressing limit.
You may experience java.lang.OutOfMemoryError: Map failed exception on 32bit JVM, if you enable this mode.
mmapFileEnableIfSupportedpublic DBMakerT mmapFileEnableIfSupported()
Enable Memory Mapped Files only if current JVM supports it (is 64bit)
I have also asked MapDB founder about that and he told me to check MapDB Code for thread interrupts.
Also a
new version of MapDB has been released