Re: Keeping mdb files opened while mmapped

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

On 12. nov. 2015 00:38, Shlomi Vaknin wrote:
> When I lsof my process, I am seeing that each mdb file is appearing twice (...)
> I know that after mmaping a file, it is not needed to be kept open, and it
> seems it is in lmdb.

By default the mmap is read-only, LDMB uses file operations for
updates.  And it keeps an extra file descriptor with the O_DSYNC or
O_SYNC flag for writing the metapage, to avoid a sync() system call.

Unless you use MDB_WRITEMAP: Then it modifies the map directly and
omits the sync descriptor, but it still needs a descriptor if the
user calls mdb_env_set_mapsize() and on Windows for mdb_env_sync().

> I know this might simply be an artifact and might not actually be a
> contributing reason for my swap ins/outs, but I wanted to hear what do you
> think about it?

If you've just written much of the database without MDB_WRITEMAP and
without sync'ing (i.e. not yet committed or you use MDB_NOSYNC),
then you'll have the new data cached for the filesystem and old data
in the map. But hopefully you didn't do that with half a gigabyte at
the same time as you worry about too high VM usage.

--
Hallvard