Re: ext3_ordered_writepage() questions

[Suparna Bhattacharya <suparna@in.ibm.com>]

On Fri, Mar 17, 2006 at 05:22:13PM -0500, Stephen C. Tweedie wrote:
> Hi,
> 
> On Fri, 2006-03-17 at 13:32 -0800, Badari Pulavarty wrote:
> 
> > I have a patch which eliminates adding buffers to the journal, if
> > we are doing just re-write of the disk block. ...
> 
> >      2.6.16-rc6      2.6.16-rc6+patch
> > real  0m6.606s        0m3.705s 
> 
> OK, that's a really significant win!  What exactly was the test case for
> this, and does that performance edge persist for a longer-running test?
> 
> > In real world, does this ordering guarantee matter ? 
> 
> Not that I am aware of.  Even with the ordering guarantee, there is
> still no guarantee of the order in which the writes hit disk within that
> transaction, which makes it hard to depend on it.
> 
> I recall that some versions of fsync depended on ordered mode flushing
> dirty data on transaction commit, but I don't think the current
> ext3_sync_file() will have any problems there.  
> 
> Other than that, the only thing I can think of that had definite
> dependencies in this are was InterMezzo, and that's no longer in the
> tree.  Even then, I'm not 100% certain that InterMezzo had a dependency
> for overwrites (it was certainly strongly dependent on the ordering
> semantics for allocates.)

Besides we seem to have already broken the guarantee in async DIO
writes for the overwrite case.

Regards
Suparna

> 
> It is theoretically possible to write applications that depend on that
> ordering, but they would be necessarily non-portable anyway.  I think
> relaxing it is fine, especially for a 100% (wow) performance gain.
> 
> There is one other perspective to be aware of, though: the current
> behaviour means that by default ext3 generally starts flushing pending
> writeback data within 5 seconds of a write.  Without that, we may end up
> accumulating a lot more dirty data in memory, shifting the task of write
> throttling from the filesystem to the VM.  
> 
> That's not a problem per se, just a change of behaviour to keep in mind,
> as it could expose different corner cases in the performance of
> write-intensive workloads.
> 
> --Stephen
> 
> 
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-- 
Suparna Bhattacharya (suparna@in.ibm.com)
Linux Technology Center
IBM Software Lab, India