When merge is in progress engine writes changes into both delta and main file. This creates additional IO load of course.
Therefore in v2.5 merge process was optimized - when page is read from delta file for merging purposes it is marked (with
BDB_merge flag) and not writed back into delta. It is OK if page not leave the cache until merge done. But there is a scenario
when this optimization is wrong :
a) page is read by merge process and marked by BDB_merge flag
b) page is changed by some user attachment and written into database file only
c) page is evicted from the page cache
d) user attachment going to change that page again
e) page is read from delta, this copy have no changes by (b) !
f) page is changed and written into both database and delta, overwriting changes by (b) in database