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     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
     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


     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
     initiated ‘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).