Re: Appropriate use of sync() from user space?

From: Chris Friesen
Date: Wed Oct 05 2011 - 20:32:44 EST

On 10/05/2011 05:57 PM, Mark Mielke wrote:

The vendor stated that sync() is integral to their synchronization
process to ensure all files reach disk before they are accessed, and
that this is not a defect in their product. We have a work around - run
"sync" before calling their command, and this generally avoids the

I think the use of sync() in this regard is a hack. According to POSIX.1
and the Linux man pages, it seems clear to me that sync() does not
guarantee data integrity (bytes guaranteed to have reached disk) - and
it also seems clear that forcing all system data to flush out in
response to a minor command is over kill. Like cutting down the forest
to harvest fruit from a single tree.

I'm wondering what you think.

Totally agree.

The susv3 man page for sync() is pretty clear: "The writing, although scheduled, is not necessarily complete upon return from sync()."

They should probably be using msync()/fsync()/fdatasync() which only affect the specified files and are supposed to wait for the data to hit the storage device. Of course this would require them to do something for each file they touch rather than once at the end of the whole operation.


Chris Friesen
Software Developer
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