Thread: Should I memory-map this file?

I have a simple database file with a fixed number of records, each with a fixed length. I don't need to insert or delete anything, I just need to read records. Also, every record is merely dereferenced with an array-like index. All this to say that I don't need to bother with complicated indexing or any 3rd party libraries like SQLite.

Furthermore, every instance of the app will read just 1 record then exit. Many instances could be running at the same time.

I see the following options.

1. Don't memory-map anything. Just seek to the target record and read it.
2. Memory-map just the record I want.
3. Memory-map the whole file.

As I understand it, the biggest reason for memory-mapping files is to dispense with the overhead of system calls. If that's the case then there seems to be little point in going with 2 or 3, since they'll have the same number of system calls as 1 (1 seeks and reads, while 2 and 3 map and unmap). However, since multiple instances will be accessing this file, I'm wondering if there might be an advantage with memory-mapping there.

If you ONLY read this file and you don't write it (or even don't write it much), consider creating a tmpfs mount (old name RAMDISC) and throw that puppy into memory. At least then you aren't doing disc reads/writes all the time.

Even if you are updating this file, you could still do this and then have a daemon that writes it back to disc every so often (like after a change is made).

Andy