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How can I get callbacks once data has been successfully written to disk in Linux?
I would like to have my programs db file mapped into memory for read/write operations and receive callbacks once a write has successfully hit disk. Kind of like what the old VMSs used to do..
You need to call fdatasync (or fsync if you really need the metadata to be synchronised as well) and wait for it to return.
You could do this from another thread, but if one thread writes to the file while another thread is doing a fdatasync(), it’s not going to be clear which of the writes are guaranteed to be persistent or not.
Databases which want to store transaction logs in a guaranteed-durable way, need to call fdatasync.
Databases (such as innodb) typically use direct IO (as well as their own data-caching, rather than rely on the OS) on their main data files, so that they know that it will be written in a predictable manner.