What does "syncing file systems..." do? ( As seen in /var/adm/messages )
At every boot, I see "syncing file systems..."
However, prior to an unknown error condition where the system will not come up from a hard boot, the previous reboot was missing this statement: "syncing file systems.."
What does this statement mean, and what is it's relevance when it is missing?
Message was edited by:
Tsungie
From the sync manpages:
<snip>SYNOPSIS
sync
DESCRIPTION
sync executes the sync system primitive. If the system is to
be stopped, sync must be called to insure file system
integrity. It will flush all previously unwritten system
buffers out to disk, thus assuring that all file modifica-
tions up to that point will be saved. See sync(2) for
details.
</snip>
From the sync(2) man pages:
<snip>
NAME
sync - update super block
SYNOPSIS
#include <unistd.h>
void sync(void);
DESCRIPTION
The sync() function writes all information in memory that
should be on disk, including modified super blocks, modified
inodes, and delayed block I/O.
Unlike fsync(3C), which completes the writing before it
returns, sync() schedules but does not necessarily complete
the writing before returning.
USAGE
The sync() function should be used by applications that
examine a file system, such as fsck(1M), and df(1M), and is
mandatory before rebooting.
</snip>
In case of a system crash, you would have to sync the buffers to disk to ensure data integrity (see above). The discrepancy (of unsync'ed disks) leads the system to start "fsck"ing the filesystem(s).
