Re: best practices WRT resizing a MDB backend?

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

On 08/28/2014 07:24 AM, Florian Weimer wrote:
> Even on Linux, sparse files which are filled incrementally can result
> in lots of fragmentation, huge extent lists (tens of thousands of
> entries longs), and long delays when opening such files.

The only "hole" is at the end.  So this shouldn't be
different from just write()ing at the end of a non-sparse
file: lmdb uses new file pages in the same order either way.

Except with put(,,data size >= 2 pages, MDB_RESERVE) and
the user filling in the item from the end forward.  Then
there will temporarily be a hole in the middle of the item.
I suppose if the user fills the item in slowly enough for
the OS to fsync, the file will get fragmented.

I suppose mdb_page_alloc() with WRITEMAP could memset new
file pages, or at least set one word in each new OS page.
I expect user programs usually fill in MDB_RESERVE items
quickly though, so hopefully it won't matter.

-- 
Hallvard