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

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Date:	Tue, 18 Oct 2011 16:53:14 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Jan Kara <jack@...e.cz>
cc:	Mark Mielke <mark@...k.mielke.cc>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Appropriate use of sync() from user space?

On Tue, 18 Oct 2011, Jan Kara wrote:

> > Quick summary: We have a vendor who is claiming that it is required
> > for their userspace program to execute sync(), and I am looking for
> > some sort of authoritative document or person to refer them to that
> > will state that this belief is incorrect and/or that this
> > architecture is not acceptable in a Unix environment.
> > 
> > I checked Google and the archives and didn't find anything
> > appropriate. Unfortunately, the word "sync" is very popular. :-)
> > 
> > We have users who have been experiencing 3 to 5 minutes "freezes"
> > for a particular command which often times out and fails. I traced
> > this down from the commercial userspace program (IBM Rational
> > ClearCase / "cleartool mkview") that they are executing to a backend
> > "view_server" process (also IBM Rational ClearCase) that is running
> > sync() as a means of synchronizing their database to disk before
> > proceeding, and VMware using a "large" memory mapped file to back
> > it's virtual "RAM". The sync() for my computer normally completes in
> > 7 to 8 seconds. The sync() for some of our users is taking 5 minutes
> > or longer. This can be demonstrated simply by typing "time sync"
> > from the command line at intervals. The time itself is relevant
> > because if it finishes before a timeout elapses - the operation
> > works (albeit slowly). If the timeout elapses, the operation fails.
> > 
> > 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 failures.
> > 
> > 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.
>   Actually the manpage is wrong. Linux waits for all data to be safely on
> disk before sync returns. So calling sync is a correct way (although
> inefficient at times) to achieve data integrity. What kernel version are
> you using? Different kernel versions are differently efficient when doing
> sync(2) and quite some effort went to make sync less prone to livelocks in
> recent kernels...
> 

Let's make sure to keep Michael Kerrisk cc'd if anything needs to be 
clarified in the manpages.
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