syncer(4) [freebsd man page]

SYNCER(4)						   BSD Kernel Interfaces Manual 						 SYNCER(4)

NAME

     syncer -- file system synchronizer kernel process

SYNOPSIS

     syncer

DESCRIPTION

     The syncer kernel process helps protect the integrity of disk volumes by flushing volatile cached file system data to disk.

     The kernel places all vnode(9)'s in a number of queues.  The syncer process works through the queues in a round-robin fashion, usually pro-
     cessing one queue per second.  For each vnode(9) on that queue, the syncer process forces a write out to disk of its dirty buffers.

     The usual delay between the time buffers are dirtied and the time they are synced is controlled by the following sysctl(8) tunable variables:

     Variable	      Default	   Description
     kern.filedelay   30	   time to delay syncing files
     kern.dirdelay    29	   time to delay syncing directories
     kern.metadelay   28	   time to delay syncing metadata

SEE ALSO

     sync(2), fsck(8), sync(8), sysctl(8)

HISTORY

     The syncer process is a descendant of the 'update' command, which appeared in Version 6 AT&T UNIX, and was usually started by /etc/rc when
     the system went multi-user.  A kernel initiated 'update' process first appeared in FreeBSD 2.0.

BUGS

     It is possible on some systems that a sync(2) occurring simultaneously with a crash may cause file system damage.	See fsck(8).

BSD
								   July 14, 2000							       BSD

SYNC(2) 						      BSD System Calls Manual							   SYNC(2)

NAME

     sync -- synchronize disk block in-core status with that on disk

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <unistd.h>

     void
     sync(void);

DESCRIPTION

     The 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
     filesystems 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

     fsync(2), dkctl(8), scsictl(8), sync(8)

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.

BSD
								  March 25, 2009							       BSD