syncer(4) file system synchronizer kernel process



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

The kernel places all vnode(9)Ns's in a number of queues. The process works through the queues in a round-robin fashion, usually processing one queue per second. For each vnode(9) on that queue, the 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 Ta Default Ta Description
kern.filedelay Ta 30 Ta time to delay syncing files
kern.dirdelay Ta 29 Ta time to delay syncing directories
kern.metadelay Ta 28 Ta time to delay syncing metadata


The process is a descendant of the `update' command, which appeared in AT&T System v6 , and was usually started by /etc/rc when the system went multi-user. A kernel initiated `update' process first appeared in Fx 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).