How to have a checkpoint file using mmap which is only synced to disk manually

Question

I need the fastest way to periodically sync file with memory.

What I think I would like is to have an mmap'd file, which is only sync'd to disk manually. I'm not sure how to prevent any automatic syncing from happening.

The file cannot be modified except at the times I manually specify. The point is to have a checkpoint file which keeps a snapshot of the state in memory. I would like to avoid copying as much as possible, since this will be need to called fairly frequently and speed is important.

Answer 1

Anything you write to the memory within a MAP_SHARED mapping of a file is considered as being written to the file at that time, as surely as if you had used write(). msync() in this sense is completely analagous to fsync() - it merely ensures that changes you have already made to the file are actually pushed out to permanent storage. You can't change this - it's how mmap() is defined to work.

In general, the safe way to do this is to write a complete consistent copy of the data to a temporary file, sync the temporary file, then atomically rename it over the prior checkpoint file. This is the only way to ensure that a crash between checkpoints doesn't leave you with an inconsistent file. Any solution that does less copying is going to require both a more complicated transaction-log style file format, and be more intrusive to the rest of your application (requiring specific hooks to be invoked in each place that the in-memory state is changed).