Thanks for the info! I have noticed that reading the database (iterating through with a cursor) the 2nd time tends to be much faster than the 1st time (sometimes the  speedup happens after the 3rd or 4th time). I presumed this was because the memory mapping happened not immediately. That's the background  of my 2. question. What was I missing? Thanks, Sam Mar 9, 2020, 01:07 by hyc(a)symas.com: > Sam Dave wrote: > >> Hi, >> >> 1. Is it possible to disable memory mapping in LMDB? >> > > No, that is fundamental to LMDB's design. The name is "Lightning Memory-Mapped Database", it means what it says. > >> The purpose of these question is to get more consistent/reproducible time benchmarks for some stuff I'm doing. >> > > There are two ways to get consistent timings - use a test DB that fits entirely in RAM, > or use a test DB that is several times larger than RAM. > > For an example of the latter, see http://www.lmdb.tech/bench/hyperdex/ > > When testing LMDB itself these tests tend to be not very interesting; they only show you > that LMDB runs as fast as your storage device has IOPS. When comparing against other DBs > these tests show how badly other DB designs fare under heavy memory pressure... > >> (As you know the drive vs. memory-mapped >> performance varies heavily.) >> > > Memory mapping has nothing to do with variance. Physical I/O determines that, and it would > be the same variance even if you were using standard read/write calls (although it would be > slower overall). > >> >> Thanks, >> Sam >> > > > -- > -- Howard Chu > CTO, Symas Corp. http://www.symas.com > Director, Highland Sun http://highlandsun.com/hyc/ > Chief Architect, OpenLDAP http://www.openldap.org/project/ >