Fwd: Re: (ITS#7713) Segmentation fault if the pagesize of the Operating system is not equal to 4096.

[Hallvard Breien Furuseth <h.b.furuseth@usit.uio.no>]

Howard Chu writes:
> Fixing this will either require adding a bunch of ugly code, or
> changing the on-disk format again. Opinions?  (...)  Adjusting
> this would require only a few minor tweaks to the code, but
> would require a full dump/reload of existing databases.

Then I hope for the format change.  But can this be macroized
without said ugliness, so existing users can compile with e.g.
-DMDB_DATA_VERSION=1 until they're prepared to dump/reload?

Still, 64K pages sounds like it can get quite wasteful.  I can
clean up my patch to support datafile pagesize != OS pagesize.
That would be a user option, not the default: Smaller datafile
pagesize means more I/O for writes. (The OS operates on pages, so
to write part of an OS page it must first read the full page.)

A problem here is that liblmdb lacks some checks for data sizes.
We can add those, but older liblmdb can break if it commits to a
datafile written by newer liblmdb and non-default sizes.  IIRC
it gets a bit hairy to support this for ldmb data format 2 but
support V1 sizes safely, so I'm either talking V2 only or just
telling users to not do this when also using an older liblmdb.

-- 
Hallvard