Applications might need to guarantee that information has been written to stable storage, or that file updates are performed in a particular order. Synchronized I/O provides for these needs.
Under SunOS, a write operation succeeds when the system ensures that all written data is readable after any subsequent open of the file. This check assumes no failure of the physical storage medium. Data is successfully transferred for a read operation when an image of the data on the physical storage medium is available to the requesting process. An I/O operation is complete when the associated data has been successfully transferred, or when the operation has been diagnosed as unsuccessful.
An I/O operation has reached synchronized I/O data integrity completion when:
For reads, the operation has been completed, or diagnosed if unsuccessful. The read is complete only when an image of the data has been successfully transferred to the requesting process. If the synchronized read operation is requested when pending write requests affect the data to be read, these write requests are successfully completed before the data is read.
For writes, the operation has been completed, or diagnosed if unsuccessful. The write operation succeeds when the data specified in the write request is successfully transferred. Furthermore, all file system information required to retrieve the data must be successfully transferred.
File attributes that are not necessary for data retrieval are not transferred prior to returning to the calling process.
Synchronized I/O file integrity completion requires that all file attributes relative to the I/O operation be successfully transferred before returning to the calling process. Synchronized I/O file integrity completion is otherwise identical to synchronized I/O data integrity completion.
fsync(3C) and fdatasync explicitly synchronize a file to secondary storage.
The fsync(3C) routine guarantees that the interface is synchronized at the I/O file integrity completion level. fdatasync guarantees that the interface is synchronized at level of I/O data integrity completion.
Applications can synchronize each I/O operation before the operation completes. Setting the O_DSYNC flag on the file description by using open(2) or fcntl(2) ensures that all I/O writes reach I/O data completion before the operation completes. Setting the O_SYNC flag on the file description ensures that all I/O writes have reached completion before the operation is indicated as completed. Setting the O_RSYNC flag on the file description ensures that all I/O reads read(2) and aio_read reach the same level of completion that is requested by the descriptor setting. The descriptor setting can be either O_DSYNC or O_SYNC.