Libelektra: mmapstorage: filter empty meta keysets

[mpranj]

Filter out empty meta keysets in mmapstorage to minimize disk and memory space used. (See discussion.)

[markus2330]

Thank you for creating this issue! If you have more spaces to fill: it would be interesting which parts use how much of the disk space.

[mpranj]

I added it to my notes. I have written quite a bit this week, but it might fit in somewhere, so thanks for the hint!

[markus2330]

As discussed, this is a research question on its own and might not be needed for this thesis.