Messages in this thread | | | Date | Tue, 18 Oct 2011 17:03:27 -0700 (PDT) | From | david@lang ... | Subject | Re: Appropriate use of sync() from user space? |
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On Tue, 18 Oct 2011, David Rientjes wrote:
> 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.
also, you may want to check if they are really doing a 'sync' (syncing the entire filesystem) or just a 'fsync' (syncing the file). Depending on the technical depth of the people you are talking to, they may say sync when what is actually happening is a fsync.
there is little dispute that fsync is correct, but not a complete answer to the issue. take a look at the LWN article on the subject at http://lwn.net/Articles/457667
Ext3 has a pathalogical condition where a sync to one file can force a complete journal flush, which isn't as bad as a sync of the entire filesystem, but can still take a long time if there is other ongoing write activity on the system (I knwo I've read about fsyncs taking longer than 30 seconds, and I think I've heard of them taking minutes). As far as I know, Ext3 is the only filesystem to suffer this problem, but unfortunantly it's the default filesystem on most linux distros.
David Lang
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