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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).