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FreeBSD 11.0 - man page for syncer (freebsd section 4)

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

     syncer -- file system synchronizer kernel process


     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 processing 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 con-
     trolled 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

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

     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 initi-
     ated 'update' process first appeared in FreeBSD 2.0.

     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
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