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From tbain98 <>
Subject Re: Replicated LevelDB : Manually copying data from one leveldb store to another
Date Tue, 04 Nov 2014 16:39:39 GMT
I've never used LevelDB, but I'd be concerned about what you're proposing
(copying the files that back a database while that database is running) no
matter what the database technology was.  If you're copying from the master
while it's active, then the master could be writing to its files while
you're doing your copy, and I doubt that anyone will tell you it's 100%
guaranteed that you won't get an inconsistent copy of the files.  Most of
the time you'll probably be fine, but I'd bet that there are race conditions
where you wouldn't be, so you're probably rolling the dice each time you do
it.  So for the process you've described, the answer is almost certainly
"don't do it, because you can't guarantee atomicity of your copy operation
and/or synchronization with the database's write operations".

I would expect better odds if you shut down the master and then copy its
files, since then you know it's not writing to them as you copy them over,
but that may or may not accomplish your goal.  (You haven't said what that
goal is, or why you can't just restart the slave with its existing LevelDB
database, so I can't speculate on what you're actually trying to do.)

But again, I'm not a LevelDB expert and it doesn't seem like there are many
of them on this mailing list, so since you want answers about the inner
workings of LevelDB, you might want to post those questions on the LevelDB
mailing list instead.

khandelwalanuj wrote
> Hi,
> Is it possible to copy data from leveldb store of one broker to another
> broker while using replicated leveldb. 
> For ex. : Let's say 3 brokers are running fine and replicating the data. I
> suddenly stop one slave broker. After some time before starting the slave
> Can I manually copy leveldb directory of master and then start the slave ?
> (I am copying each and every file other than the nodeid.txt) 
> In my test setup it is working fine but not sure if it will fail in some
> corner cases. Let me know if some issues are there with this. 
> Thanks,
> Anuj

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