Question

I’m working on a ACID database software product and I have some questions about file durability on WinOS.

CreateFile has two attributes, FILE_FLAG_WRITE_THROUGH and FILE_FLAG_NO_BUFFERING – do I need both these to achieve file durability (ie. override all kinds of disk or OS file caching)? I’m asking since they seem to do the same thing, and setting FILE_FLAG_NO_BUFFERING causes WriteFile to throw an ERROR_INVALID_PARAMETER error.

Answer 1

FILE_FLAG_NO_BUFFERING specifies no caching at al. No read nor write cache all data goes directly to and from your application to disk. This is mostly usefull if you read such large chunks that caching is useless or you do your own caching. Note WhozCraig’s comment on properly aligning your data when using this flag.

FILE_FLAG_WRITE_THROUGH only means that writes should written directly to disk before the function returns. This is enough to achieve ACID while it still gives the option to the OS to cache data from the file.

Using FlushFileBuffers() can provide a more efficient approach for achieving ACID as you can do several writes to a file and then flush them in one go. Combining writes in one flush is very important as non cached writes will limit you to the spindle speed of your harddrive. 120 non cached writes or flushes per second max for a 7200 rpm disk.