Re: [RFC] ext3: per-process soft-syncing data=ordered mode

[Chris Snook <csnook@redhat.com>]

Al Boldi wrote:
> Greetings!
> 
> data=ordered mode has proven reliable over the years, and it does this by 
> ordering filedata flushes before metadata flushes.  But this sometimes 
> causes contention in the order of a 10x slowdown for certain apps, either 
> due to the misuse of fsync or due to inherent behaviour like db's, as well 
> as inherent starvation issues exposed by the data=ordered mode.
> 
> data=writeback mode alleviates data=order mode slowdowns, but only works 
> per-mount and is too dangerous to run as a default mode.
> 
> This RFC proposes to introduce a tunable which allows to disable fsync and 
> changes ordered into writeback writeout on a per-process basis like this:
> 
>       echo 1 > /proc/`pidof process`/softsync
> 
> 
> Your comments are much welcome!

This is basically a kernel workaround for stupid app behavior.  It wouldn't be 
the first time we've provided such an option, but we shouldn't do it without a 
very good justification.  At the very least, we need a test case that 
demonstrates the problem and benchmark results that prove that this approach 
actually fixes it.  I suspect we can find a cleaner fix for the problem.

	-- Chris