The Universe of Disco

Corrections about sync(2)
I made some errors in today's post about sync and fsync.

Most important, I said that "the sync() system call marks all the kernel buffers as dirty". This is totally wrong, and doesn't even make sense. Dirty buffers are those with data that needs to be written out. Marking a non-dirty buffer as dirty is a waste of time, since nothing has changed in the buffer, but it will now be rewritten anyway. What sync() does is schedule all the dirty buffers to be written as soon as possible.

On some recent systems, sync() actually waits for all the dirty buffers to be written, and a bunch of people tried to correct me about this. But my original article was right: historically, it was not so, and even today it's not universally true. In former times, sync() would schedule the buffers for writing, and then return before the data was actually written.

I said that one of the duties of init was to call sync() every thirty seconds, but this was mistaken. That duty actually fell to a separate program, known as update. While discussing this with one of the readers who wrote to correct me, I looked up the source for Version 7 Unix, to make sure I was right, and it's so short I thought I might as well show it here:

        /*
         * Update the file system every 30 seconds.
         * For cache benefit, open certain system directories.
         */

        #include <signal.h>

        char *fillst[] = {
                "/bin",
                "/usr",
                "/usr/bin",
                0,
        };

        main()
        {
                char **f;

                if(fork())
                        exit(0);
                close(0);
                close(1);
                close(2);
                for(f = fillst; *f; f++)
                        open(*f, 0);
                dosync();
                for(;;)
                        pause();
        }

        dosync()
        {
                sync();
                signal(SIGALRM, dosync);
                alarm(30);
        }

In various systems more recent than V7, the program was known by various names, but it was update for a very long time.

Several people wrote to correct me about the:

        # sync
        # sync
        # sync
        # halt

Nobody disputed my contention that Linus was suffering from the promptings of the Evil One when he tried to change the semantics of fsync(), and nobody seems to know the proper name of the false god of false efficiency. I'll give this some thought and see what I can come up with.

Thanks to Tony Finch, Dmitry Kim, and Stefan O'Rear for discussion of these points.

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