8    File Synchronization

By default, UNIX systems read from and write to a buffer cache that is kept in memory, and avoid actually transferring data to disk until the buffer is full, or until the application calls a sync function to flush the buffer cache. This increases performance by avoiding the relatively slow mechanical process of writing to disk more often then necessary.

Realtime input and output operations are of two types:

• Asynchronous I/O, which frees the application to perform other tasks while input is written or read (see Chapter 7)

• Synchronized I/O, which performs the write or read operation and verifies its completion before returning

DIGITAL UNIX supports POSIX 1003.1b file synchronization for the UFS and AdvFS file systems, as described in this chapter. However, DIGITAL recommends use of the UFS file system for better realtime performance.

Synchronized I/O is useful when the integrity of data and files is critical to an application. Synchronized output assures that data that is written to a device is actually stored there. Synchronized input assures that data that is read from a device is a current image of data on that device.

Two levels of file synchronization are available, data integrity and file integrity:

• Data integrity

  • Write operations: data in the buffer is transferred to disk, along with file system information necessary to retrieve the data.

  • Read operations: any pending write operations relevant to the data being read complete with data integrity before the read operation is performed.

• File integrity

  • Write operations: data in the buffer and all file system information related to the operation are transferred to disk.

  • Read operations: any pending write operations relevant to the data being read complete with file integrity before the read operation is performed.

8.1    How to Assure Data or File Integrity

You can assure data integrity or file integrity at specific times by using function calls, or you can set file status flags to force automatic file synchronization for each read or write call associated with that file.

Use of synchronized I/O may degrade system performance; see Chapter 11.

8.1.1    Using Function Calls

You can choose to write to buffer cache as usual, and call functions explicitly when you want the program to flush the buffer to disk. For instance, you may want to use the buffer cache when a lot of I/O is occurring, and call these functions when activity slows down. Two functions are available:

Refer to online reference pages for a complete description of these functions.

8.1.2    Using File Descriptors

If you want to write data to disk in all cases automatically, you can set file status flags to force this behavior instead of making explicit calls to fdatasync or fsync.

To set this behavior, use these flags with the open or fcntl function:

fd = open("my_file", O_RDWR|O_CREAT|O_DSYNC, 0666);

fd = open("my_file", O_RDWR|O_CREAT|O_SYNC, 0666);

fd = open("my_file", O_RDWR|O_CREAT|O_SYNC|O_RSYNC, 0666);
fd = open("my_file", O_RDWR|O_CREAT|O_DSYNC|O_RSYNC, 0666);

If both the O_DSYNC and O_SYNC flags are set using the open or fcntl function, O_SYNC takes precedence.