Re: (ITS#7789) Unreliable mdb_env_set_mapsize()

[hyc@symas.com]

h.b.furuseth@usit.uio.no wrote:
> Another problem:
>
> With an MDB_RDONLY environment on Windows, env->me_mapsize
> does not seem to affect the actual map size or vice versa.
> mdb_env_map() does
> 		/* Don't set explicit map size, use whatever exists */
>
> If the user sets a mapsize bigger than the current file,

That is impossible on Windows. The OS will not allow a mapsize bigger than the 
filesize.

> and another process resizes the map and grows the DB into
> the new size, then the RDONLY process will not discover
> in mdb_txn_renew0() that its map is too small.
>
> If there is a technical reason for the current behavior,

That change was made to accomodate opening of an mdb_copy'd DB file in 
read-only mode. Since mdb_copy only copies the used pages, the resulting file 
will be smaller than the configured mapsize. If you attempt to map this file 
as-is with the configured mapsize, it will fail. You would have to resize the 
file, but since the env is opened read-only, you should not make any such 
change to the file.

We could alternatively change mdb_copy to explicitly set the filesize to match 
the mapsize at the end of the copy, and then this particular hack wouldn't be 
needed.

> maybe it could create the full map (for the address space)
> but record the smaller mapped size in MDB_env, and then
> mdb_txn_renew0 could do some mutex-protected magic to
> map in the new file segment if the file grew.  That
> should avoid surprising the user.

-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP  http://www.openldap.org/project/