| SYNCER(4) | Device Drivers Manual | SYNCER(4) |
syncer — file
system synchronizer kernel process
syncer |
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 |
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).
| July 14, 2000 | Debian |