NAME
sync —
synchronize disk block in-core status
with that on disk
LIBRARY
library “libc”
SYNOPSIS
#include
<unistd.h>
void
sync(void);
DESCRIPTION
Thesync()
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 ioflush ensures
that dirty buffers are synced to disk eventually. By default, a dirty buffer
is synced after 30 seconds, but some file systems exploit
ioflush features to sync directory data and metadata
faster (after 15 and 10 seconds, respectively).
The function fsync(2) may be used to synchronize individual file descriptor attributes.
CAUTIONS
Many modern disks contain write-back caches. In theory
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.
It may also be desirable to use dkctl(8) or scsictl(8) to disable the write-back cache entirely.
SEE ALSO
HISTORY
A sync() function call appeared in
Version 6 AT&T UNIX.
Historically, 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 NetBSD,
sync() does not return until all buffers have been
written.