Is there any way to speed up frequent file write operations? CFile.

Question

My task is to write small parts of data to file frequently and guaranteed. It's a some type of logging system, that must provide a guaranteed safety and speed of saving data to disk. I found that brakes appear in the Flush() function that is called too often. So, I decided to get rid of it.

I used flags, that prohibit buffering and caching data (osNoBuffer and writeThrough). After this, as I understand, I could perform file I/O operations (write data to file) without use of flush()? Another one question. How can I understand that data is definitely written to disk without caching and buffering? Is it The wright way to do what I want or may be the other way to speed up frequent file write operations??

Answer 1

flush() takes so long, precisely because it establishes the guarantee you're looking for. Files are usually written to disk, which (probably) are spinning rusty disks. Files are physically scattered over the surface of these disks, and it takes milliseconds to move the read/write head to the right place. 50 flushes per second means an average of 20 milliseconds per flush, which is close to what your disk mechanically can do.

Without flush, your writes are combined so multiple writes will require only one movement of the read/write head. Without even examining the specific flags, that must necessarily mean that data is temporarily at risk.

In software alone, you can never know whether data is truly written to disk. We know this because of research performed by Sun, when developing the ZFS file system. It turns out that cheaper, slower disks lie about flushing, and return before data is actually flushed. You will need server-grade disks to guarantee this.

SSDs perform quite well in this area. They have no need to physically move write heads. Therefore, there's less incentive for them to lie about flushes.