xref: /minix3/lib/libc/sys/sync.2 (revision 2fe8fb192fe7e8720e3e7a77f928da545e872a6a)

.\"	$NetBSD: sync.2,v 1.17 2009/03/25 06:46:21 wiz Exp $
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.\"     @(#)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.