when data it is updated it writes the whole record to the file

That's a nice compromise for data integrity purposes.

A possible enhancement, as your data is in shared memory, would be to start another background thread with a queue and offload the write-back to disk from your processing threads, by queueing the primary key (filename) of updated records.

When the program loads it scans the directory and reads all records in memory. At this time I do not need sharing between processes

As long as your data continues to fit in memory, and your only operating on it from one processor that seems like an effective strategy.