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(ITS#7725) LMDB readahead should be configurable

Full_Name: Howard Chu
Version: 2.4.35+
OS: Linux
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (
Submitted by: hyc

In 2.4.35 (commit b1da555c4c7275d7f756d693c42814388a9aa839) code was added to
unconditionally turn off readahead, because it is harmful to random read
performance when a DB is larger than RAM and/or system RAM is already full.
Unfortunately turning it off harms sequential read performance. It should be
configurable, since different behaviors are needed in different use cases.