ovsqlite: add mmapsize configuration

[kev009]

No description provided.

[Julien-Elie]

Thanks!
I'll commit it soon.

[Julien-Elie]

A good starting value is the size of the database file or the amount of available memory, whichever is smaller.

Is it safe to recommend to set the amount of available memory? It may starve the server. Shouldn't something be said about that?
I read in https://www.sqlite.org/mmap.html that performance does not always increase with memory-mapped I/O, and that any I/O error on a memory-mapped file cause a signal which, if not caught by the application, results in a program crash.
Maybe the SQL requests we use all behave faster with mmap, so we're not in the above performance caveat.

[kev009]

Yes it is generally safe to mmap any sized file (subject to virtual address space limitations, on i.e. 32b). The kernel will page in and out the file as needed so the fixed memory use at any point is low. In a multi-process architecture it is ideal, i.e. ovsqlite-server and nnrpd direct readers.

Of the caveats, the Unified Buffer Cache one is the only critical and applies to all the other places inn uses mmap. It is not the right syscall for ZFS because they never correctly integrated it with any OS memory management.