SYNC(2) | System Calls Manual | SYNC(2) |
sync
—
#include <unistd.h>
void
sync
(void);
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 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.
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.
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.
March 25, 2009 | NetBSD 9.2 |