LMDB physical file size

[Christian Sell <christian@gsvitec.com>]

Hello,

we are using LMDB as the underlying storage engine for a lightweight + high
performance special-purpose object + mass data database. I have 2 questions
about the size of the physical file used by LMDB:

To create the environment, we are using a mapsize of 1 GiB and the flags
MDB_NOSUBDIR | MDB_NOLOCK. Under Linux, this results in one file with a size
that seems to correspond to the size of the data actually stored. However, under
Windows, the file size is the same as the mapsize, namely 1 GiB. We are
currently using env_copy2 and MDB_COMPACT to push this down every time the env
is closed, but I fear that this will become very slow with large databases.

The same issue surfaced under Linux when we were recently experimenting with the
MDB_WRITEMAP option to improve performance when dealing with very large data
sets. This option caused also the Linux file size to go up to 1 GiB, even though
the actual data was < 50 K.

We'd like to hear if there are ways to improve this.

thanks,
Christian