129 lines
4.1 KiB
Groff
129 lines
4.1 KiB
Groff
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.\" $NetBSD: fsync.2,v 1.4 1995/02/27 12:32:38 cgd Exp $
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.\"
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.\" Copyright (c) 1983, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)fsync.2 8.1 (Berkeley) 6/4/93
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.\"
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.Dd June 4, 1993
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.Dt FSYNC 2
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.Os BSD 4.2
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.Sh NAME
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.Nm fsync
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.Nd "synchronize a file's in-core state with that on disk"
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.Sh SYNOPSIS
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.Fd #include <unistd.h>
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.Ft int
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.Fo fsync
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.Fa "int fildes"
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.Fc
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.Sh DESCRIPTION
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.Fn fsync
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causes all modified data and attributes of
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.Fa fildes
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to be moved to a permanent storage device.
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This normally results in all in-core modified copies
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of buffers for the associated file to be written to a disk.
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.Pp
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Note that while
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.Fn fsync
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will flush all data from the host to the drive
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(i.e. the "permanent storage device"),
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the drive itself may not physically write the data
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to the platters for quite some time
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and it may be written in an out-of-order sequence.
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.Pp
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Specifically, if the drive loses power
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or the OS crashes,
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the application may find that only some or none of their data was written.
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The disk drive may also re-order the data
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so that later writes may be present, while earlier writes are not.
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.Pp
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This is not a theoretical edge case.
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This scenario is easily reproduced with real world workloads
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and drive power failures.
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.Pp
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For applications that require tighter guarantees
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about the integrity of their data,
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Mac OS X provides the F_FULLFSYNC fcntl.
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The F_FULLFSYNC fcntl asks the drive to flush all buffered data
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to permanent storage.
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Applications, such as databases,
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that require a strict ordering of writes
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should use F_FULLFSYNC to ensure that their data
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is written in the order they expect.
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Please see
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.Xr fcntl 2
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for more detail.
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.Pp
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.Sh RETURN VALUES
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.Rv -std fsync
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.Sh ERRORS
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The
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.Fn fsync
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system call will fail if:
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.Bl -tag -width Er
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.\" ==========
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.It Bq Er EBADF
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.Fa fildes
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is not a valid descriptor.
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.\" ==========
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.It Bq Er EINTR
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Its execution is interrupted by a signal.
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.\" ==========
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.It Bq Er EINVAL
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.Fa fildes
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refers to a file type (e.g., a socket)
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that does not support this operation.
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.\" ==========
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.It Bq Er EIO
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An I/O error occurred while reading from or writing to the file system.
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.El
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.Pp
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If a queued I/O operation fails,
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.Fn fsync
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may fail with any of the errors defined for
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.Xr read 2
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or
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.Xr write 2 .
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.Sh SEE ALSO
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.Xr fcntl 2 ,
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.Xr read 2 ,
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.Xr sync 2 ,
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.Xr write 2 ,
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.Xr sync 8 ,
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.Xr update 8
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.Sh HISTORY
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The
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.Fn fsync
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function call appeared in
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.Bx 4.2 .
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