Hello, we are using LMDB as the underlying storage engine for a lightweight + high performance special-purpose object + mass data database. I have 2 questions about the size of the physical file used by LMDB: To create the environment, we are using a mapsize of 1 GiB and the flags MDB_NOSUBDIR | MDB_NOLOCK. Under Linux, this results in one file with a size that seems to correspond to the size of the data actually stored. However, under Windows, the file size is the same as the mapsize, namely 1 GiB. We are currently using env_copy2 and MDB_COMPACT to push this down every time the env is closed, but I fear that this will become very slow with large databases. The same issue surfaced under Linux when we were recently experimenting with the MDB_WRITEMAP option to improve performance when dealing with very large data sets. This option caused also the Linux file size to go up to 1 GiB, even though the actual data was < 50 K. We'd like to hear if there are ways to improve this.

On 09/11/15 18:47, Christian Sell wrote: > To create the environment, we are using a mapsize of 1 GiB and the flags > MDB_NOSUBDIR | MDB_NOLOCK. Under Linux, this results in one file with a > size that seems to correspond to the size of the data actually stored. > However, under Windows, the file size is the same as the mapsize, namely > 1 GiB. (...) The same issue surfaced under Linux (...) with the > MDB_WRITEMAP option That's the logical size, which can be bigger than the physical size. In lmdb's case, the end of the file doesn't use any disk space. On filesystems which support this, anyway. Most do. So, nevermind mdb_copy - there is no problem to fix. On Unix, 'du <file>' shows disk usage. Don't know about Windows. When you want to copy the file anyway, you should use mdb_copy rather than plain filecopy. And MDB_COMPACT does shrink the file somewhat since it drops pages which LMDB has freed and not yet reused, but that's another matter. The DB would grow later anyway, LDMB does need pages it can write to.

>>> Hallvard Breien Furuseth <h.b.furuseth(a)usit.uio.no&gt; schrieb am 09.11.2015 um 23:00 in Nachricht <56411792.8080205(a)usit.uio.no&gt;: > On 09/11/15 18:47, Christian Sell wrote: >> To create the environment, we are using a mapsize of 1 GiB and the flags >> MDB_NOSUBDIR | MDB_NOLOCK. Under Linux, this results in one file with a size >> that seems to correspond to the size of the data actually stored. However, > under >> Windows, the file size is the same as the mapsize, namely 1 GiB. >> (...) The same issue surfaced under Linux (...) with the MDB_WRITEMAP option > > That's the logical size, which can be bigger than the physical > size. In lmdb's case, the end of the file doesn't use any disk > space. On filesystems which support this, anyway. Most do. > So, nevermind mdb_copy - there is no problem to fix. > > On Unix, 'du <file>' shows disk usage. Don't know about Windows. > > When you want to copy the file anyway, you should use mdb_copy > rather than plain filecopy. And MDB_COMPACT does shrink the file > somewhat since it drops pages which LMDB has freed and not yet > reused, but that's another matter. The DB would grow later > anyway, LDMB does need pages it can write to. I wonder as SSD become more and more common: Should LMDB have a way to