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Re: Fwd: Using lmdb as a pure in memory store

Russell Haley wrote:

Hello,

I wasn't fully subscribed when I sent this so I'll send it again.

Any input or reference hints would be great. I love reading manuals. :)


http://lmdb.tech/doc/


Russ


---------- Forwarded message ----------
From: Russell Haley <russ.haley@gmail.com>
Date: Tue, Sep 20, 2016 at 4:52 PM
Subject: Using lmdb as a pure in memory store
To: openldap-technical@openldap.org


Hi there,

We are currently evaluating in memory key value stores for ~100,000 -
200,000 records in an embedded system. I suggested lmdb but it is
being discounted for some reasons I thought I'd validate:

1) It is currently thought that a on disk file is REQUIRED for the
system. Does MDB_NOSYNC turn off the disk caching? Can it be run as a
pure in-memory database? Could the file not just be mapped to a
memdisk?
A file is required. Whether the file is on-disk or not is irrelevant. It can of course be on a RAMdisk (or, in Linux, a tmpfs), which would then make it pure in-memory. 

MDB_NOSYNC turns off syncing, that's why it is named that.


2) Because these values can come very fast, that the use of a lock
file would cause delay and too much wear on the nand based disk (SSD).


No. That's not how the lock file is used.


I see a no locking option ( MDB_NOLOCK) that would stop a lock file
being written.
This option should only be used if you're implementing your own lock manager. Hint - no matter what approach you use, any other lock manager you use will be slower than LMDB's. 


Again, another option would be mapping the lock file to
a memdisk to  handle that?


Yes.

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