Data consistency and durability (The GNU Fortran Compiler) (original) (raw)

4.6 Data consistency and durability ¶

This section contains a brief overview of data and metadata consistency and durability issues when doing I/O.

With respect to durability, GNU Fortran makes no effort to ensure that data is committed to stable storage. If this is required, the GNU Fortran programmer can use the intrinsic FNUM to retrieve the low level file descriptor corresponding to an open Fortran unit. Then, using e.g. the ISO_C_BINDING feature, one can call the underlying system call to flush dirty data to stable storage, such asfsync on POSIX, _commit on MinGW, or fcntl(fd, F_FULLSYNC, 0) on macOS. The following example shows how to call fsync:

! Declare the interface for POSIX fsync function interface function fsync (fd) bind(c,name="fsync") use iso_c_binding, only: c_int integer(c_int), value :: fd integer(c_int) :: fsync end function fsync end interface

! Variable declaration integer :: ret

! Opening unit 10 open (10,file="foo")

! ... ! Perform I/O on unit 10 ! ...

! Flush and sync flush(10) ret = fsync(fnum(10))

! Handle possible error if (ret /= 0) stop "Error calling FSYNC"

With respect to consistency, for regular files GNU Fortran uses buffered I/O in order to improve performance. This buffer is flushed automatically when full and in some other situations, e.g. when closing a unit. It can also be explicitly flushed with theFLUSH statement. Also, the buffering can be turned off with theGFORTRAN_UNBUFFERED_ALL andGFORTRAN_UNBUFFERED_PRECONNECTED environment variables. Special files, such as terminals and pipes, are always unbuffered. Sometimes, however, further things may need to be done in order to allow other processes to see data that GNU Fortran has written, as follows.

The Windows platform supports a relaxed metadata consistency model, where file metadata is written to the directory lazily. This means that, for instance, the dir command can show a stale size for a file. One can force a directory metadata update by closing the unit, or by calling _commit on the file descriptor. Note, though, that _commit forces all dirty data to stable storage, which is often a very slow operation.

The Network File System (NFS) implements a relaxed consistency model called open-to-close consistency. Closing a file forces dirty data and metadata to be flushed to the server, and opening a file forces the client to contact the server in order to revalidate cached data. fsync also forces a flush of dirty data and metadata to the server. Similar to open and close, acquiring and releasing fcntl file locks, if the server supports them, also forces cache validation and flushing dirty data and metadata.