[jimis]

LMDB is a very good choice for many well-known reasons. I don't need to expand here, the advantages are well documented, and more and more projects are choosing LMDB.

However LMDB does not solve all problems, and can be a bad choice for some, and I couldn't find this documented anywhere. Specifically write-intensive workload. Why?

- LMDB by default provides full ACID semantics, which means that after every key-value write committed, it needs to sync to disk. Apparently if this happens tens of times per second, your system performance will suffer.

- LMDB provides a super-fast asynchronous mode (`MDB_NOSYNC`), and this is the one most often benchmarked. Writes are super-fast with this. But a little known fact is that you lose all of ACID, meaning that a system crash can cause total loss of the database. Only use `MDB_NOSYNC` if your data is expendable.

In short, I would advise against LMDB if you are expecting to have more than a couple of independent writes per second. In this case, consider choosing a database that syncs to disk only occasionally, offering just ACI semantics (without Durability, which means that a system crash can cause loss of only the last seconds of data).

[boardwaalk]

> But a little known fact is that you lose all of ACID, meaning that a system crash can cause total loss of the database. Only use `MDB_NOSYNC` if your data is expendable.

Last I looked into LMDB, this was only the case if the filesystem doesn't respect write ordering, which depends on the filesystem. Otherwise you get everything but durability (i.e. ACI) If I recall, writes are ordered by default on Ext3.

[dikei]

This is exactly our experience. Using the default settings, RocksDB massively outperform LMDB on single-key write workload, because it writes asynchronously.

[hyc_symas]

Your advice made sense in the age of rotating platter HDDs, limited to a max of ~120 IOPS. Today's world of NVMe SSDs makes your considerations obsolete.

[JulianMorrison]

There's an even older technology, battery backed RAM cached HDDs, that gives you everything an SSD can, except the thing you aren't actually using here, fast random-access read performance.

[blaisio]

That's not true. With SSDs we can sync with the disk more often, but it's still very slow.