[Bugs] [Bug 1393419] read-ahead not working if open-behind is turned on

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1393419



--- Comment #18 from Raghavendra G <rgowdapp at redhat.com> ---
(In reply to Milind Changire from comment #15)
> (In reply to Raghavendra G from comment #13)
> > We can see that performance gets affected by the number of read-ahead page
> > count in both scenarios - with/without fix. 

without fix, there is some percentage of opens where open-behind actually winds
down and hence read-ahead will be working for those fds. I guess variance due
to change of page-count can be attributed to this.

Also, note that as found on bz 1214489, reads are interspersed with getattr.
So, the cache is thrown away. With large value of page-count more pages are
wasted and hence performance degrades for larger value of page count.

I verified from logs attached that patch https://review.gluster.org/15811 makes
sure that all opens are witnessed by read-ahead and hence a fix to current bug.

To summarize patch #15811 is a necessary fix and solves the problem at hand.
Problem with read-ahead performance are tracked by other bzs like bz 1214489.

> > Milind will do the same tests by
> > setting client log-level to TRACE and attach logs to the bz. Hence setting
> > needinfo on him.
> > 
> > Once logs are attached, csaba has agreed to take the ownership of bz. Hence
> > assigning the bz to csaba.
> 
> Perf numbers for the TRACEL level logging test run are available as
> attachments.
> 
> TRACE level logs are available at:
> 1.
> https://nofile.io/f/is0XjWnaLFK/bz-1393419-test-run-without-patch-client-
> TRACE.log.gz
> 2.
> https://nofile.io/f/Oh3lkzaxMSe/bz-1393419-test-run-with-patch-client-TRACE.
> log.gz

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