minix/lib/libc/sys/sync.2
Ben Gras 2fe8fb192f Full switch to clang/ELF. Drop ack. Simplify.
There is important information about booting non-ack images in
docs/UPDATING. ack/aout-format images can't be built any more, and
booting clang/ELF-format ones is a little different. Updating to the
new boot monitor is recommended.

Changes in this commit:

	. drop boot monitor -> allowing dropping ack support
	. facility to copy ELF boot files to /boot so that old boot monitor
	  can still boot fairly easily, see UPDATING
	. no more ack-format libraries -> single-case libraries
	. some cleanup of OBJECT_FMT, COMPILER_TYPE, etc cases
	. drop several ack toolchain commands, but not all support
	  commands (e.g. aal is gone but acksize is not yet).
	. a few libc files moved to netbsd libc dir
	. new /bin/date as minix date used code in libc/
	. test compile fix
	. harmonize includes
	. /usr/lib is no longer special: without ack, /usr/lib plays no
	  kind of special bootstrapping role any more and bootstrapping
	  is done exclusively through packages, so releases depend even
	  less on the state of the machine making them now.
	. rename nbsd_lib* to lib*
	. reduce mtree
2012-02-14 14:52:02 +01:00

102 lines
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Groff

.\" $NetBSD: sync.2,v 1.17 2009/03/25 06:46:21 wiz Exp $
.\"
.\" Copyright (c) 1980, 1991, 1993
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" SUCH DAMAGE.
.\"
.\" @(#)sync.2 8.1 (Berkeley) 6/4/93
.\"
.Dd March 25, 2009
.Dt SYNC 2
.Os
.Sh NAME
.Nm sync
.Nd "synchronize disk block in-core status with that on disk"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft void
.Fn sync void
.Sh DESCRIPTION
The
.Fn sync
function forces a write of dirty (modified) buffers
in the block buffer cache out
to disk.
The kernel keeps this information in core to reduce
the number of disk I/O transfers required by the system.
As information in the cache is lost after a system crash,
kernel thread
.Nm ioflush
ensures that dirty buffers are synced to disk
eventually.
By default, a dirty buffer is synced after 30 seconds,
but some filesystems exploit
.Nm ioflush
features to sync directory data and metadata faster
(after 15 and 10 seconds, respectively).
.Pp
The function
.Xr fsync 2
may be used to synchronize individual file descriptor
attributes.
.Sh CAUTIONS
Many modern disks contain write-back caches.
In theory
.Fn sync
flushes these.
In practice there are many possible ways for this mechanism to go
astray.
It is prudent (where possible) to allow a few seconds after syncing
for everything to settle before e.g. turning off the power.
.Pp
It may also be desirable to use
.Xr dkctl 8
or
.Xr scsictl 8
to disable the write-back cache entirely.
.Sh SEE ALSO
.Xr fsync 2 ,
.Xr dkctl 8 ,
.Xr scsictl 8 ,
.Xr sync 8
.Sh HISTORY
A
.Fn sync
function call appeared in
.At v6 .
.Pp
Historically,
.Fn sync
would schedule buffers for writing but not actually wait for the
writes to finish.
It was necessary to issue a second or sometimes a third call to ensure
that all buffers had in fact been written out.
In
.Nx ,
.Fn sync
does not return until all buffers have been written.