SYNCER(4)

NAME
syncer - filesystem synchronizer kernel process

SYNOPSIS
syncer

DESCRIPTION
The syncer kernel process helps protect the integrity of disk volumes by flushing volatile cached filesystem 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

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

HISTORY
The syncer process is a descendant of the ‘update’ command, which appeared in 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).