Low-hanging LMDB fruits

[miodvallat]

Short description

This simple PR intends to improve the LMDB backend performance in the following situations:

• upon low-memory conditions, by disabling operating system read ahead on the database file, which could cause some read operations to put database contents in the buffer cache at the expense of memory available to the processes.
• when processing large RRSET updates, by allocating a temporary data structure once to its desired size, rather than having it grow multiple times to reach its final size.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[rgacogne]

Code looks good! Do you know if setting MDB_NORDAHEAD could actually lower performance in some cases (enough RAM for the DB to be loaded in memory)?

[miodvallat]

Do you know if setting MDB_NORDAHEAD could actually lower performance in some cases (enough RAM for the DB to be loaded in memory)?

I don't know. This will depend on the workload and the way the database is accessed. Of course, once the whole database is loaded in memory, this doesn't matter anymore 😁

[miodvallat]

(also the LMDB documentation does not mention any horrible scenario arising from the use this option)