Synchronized I/O

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.

Modes of Synchronization

Under SunOS 5.0 through 5.8, for a write operation, data is successfully transferred to a file when the system ensures that all written data is readable after any subsequent open of the file in the absence of a failure of the physical storage medium (even one that follows a system or power failure). For a read operation data is successfully transferred when an image of the data on the physical storage medium is available to the requesting process. An I/O operation is complete when either the associated data has been successfully transferred or 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 unsuccessful. The read is complete only when an image of the data has been successfully transferred to the requesting process. If there were any pending write requests affecting the data to be read at the time that the synchronized read operation was requested, these write requests are successfully transferred prior to reading the data.

For writes, the operation has been completed or diagnosed if unsuccessful. The write is complete only when the data specified in the write request is successfully transferred, and all file system information required to retrieve the data is successfully transferred.

File attributes that are not necessary for data retrieval (access time, modification time, status change time) are not transferred prior to returning to the calling process.

Synchronized I/O file integrity completion is identical to synchronized I/O data integrity completion with the addition that all file attributes relative to the I/O operation (including access time, modification time, status change time) must be successfully transferred prior to returning to the calling process.

Synchronizing a File

The fsync(3C) and fdatasync(3RT) functions explicitly synchronize a file to secondary storage.

fsync(3C) guarantees the function is synchronized at the I/O file integrity completion level, while fdatasync(3RT) guarantees the function is synchronized at the I/O data integrity completion level.

Applications can synchronize each I/O operation before the operation completes. Setting the O_DSYNC flag on the file description by open(2) or fcntl(2) ensures that all I/O writes (write(2) and aiowrite(3AIO)) have reached I/O data completion before the operation is indicated as completed. 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(3RT)) have reached the same level of completion as request for writes by the setting, O_DSYNC or O_SYNC, on the descriptor.