From my totally biased perspective, MDBM is utter garbage. They use mmap but make absolutely zero effort to use it safely. This was the biggest obstacle to overcome in developing LMDB; I had a few lengthy conversations with the SleepyCat guys about it as well. It's the reason it took 2 years (from 2009 when we first started talking about it, to 2011 first code release) to get LMDB implemented. If you want to call something a "database" you have to do more than just mmap a file and start shoving data into it - you have to exert some kind of control over how and when the mapped data gets persisted to disk. Otherwise, if you just let the OS randomly flush things, you'll wind up with garbage. As Keith Bostic said to me (private email):
"The most significant problem with building an mmap'd back-end is implementing write-ahead-logging (WAL). (You probably know this, but just in case: the way databases usually guarantee consistency is by ensuring that log records describing each change are written to disk before their transaction commits, and before the database page that was changed. In other words, log record X must hit disk before the database page containing the change described by log record X.)
In Berkeley DB WAL is done by maintaining a relationship between the database pages and the log records. If a database page is being written to disk, there's a look-aside into the logging system to make sure the right log records have already been written. In a memory-mapped system, you would do this by locking modified pages into memory (mlock), and flushing them at specific times (msync), otherwise the VM might just push a database page with modifications to disk before its log record is written, and if you crash at that point it's all over but the screaming."
The harsh realities of working with mmap are what dictated LMDB's copy-on-write design - it's the only way to ensure consistency with an mmap without losing performance (due to multiple mlock/msync syscalls). None of these design considerations are evident in MDBM.
LMDB's mmap is read-only by default, because otherwise it's trivial to permanently corrupt a database by overwriting a record, writing past the end, etc. MDBM's mmap is read-write, and the only "protection" you get is a doc that tells you "be Vewwy vewwy careful!" Ridiculously sloppy.
LMDB's design and implementation are proven incorruptible. MDBM (and LevelDB and all its derivatives) are proven to be quite fragile. https://www.usenix.org/conference/osdi14/technical-sessions/...
Leaving reliability aside for a moment, there's also the issue of performance and efficiency. We used to use DBM-style hashes for the indexes in OpenLDAP, up to release 2.1. We abandoned them in favor of B-trees in OpenLDAP 2.2 because extensive benchmarking showed that BDB's B-trees were faster than its hash implementation at very large data sizes. The fundamental problem is that hash data structures are only fast when they are sparsely populated. When the number of data records you need to work with increases to fill the table, you start getting more and more hash collisions that result in lots of linear probes (or whatever other hash recovery strategy you're using). The other problem is that the very sparse/unordered nature of hashes makes them extremely cache unfriendly - you get zero locality-of-reference for groups of related queries. So as your data volumes increase, you get less and less benefit from the amount of RAM you have available. When the data exceeds the size of RAM, the number of disk seeks required for an arbitrary lookup is enormous, and every read is a random access. Using a hash for a large-scale data store is just horrible. (We tested this extensively a decade ago http://www.openldap.org/lists/openldap-devel/200401/msg00077... )