improving concurrency/performance (fwd)

[Joshua Schmidlkofer]

> 
> Yes, on ext3, an fsync() syncs the entire filesystem.  It has to, because
> all the metadata for each file is shared - it's just a string of
> journallable blocks.  Similar story with the data, in ordered mode.
> 
> So effectively, fsync()ing five files one time each is performing 25 fsync()s.
> 
> One fix (which makes the application specific to ext3 in ordered-data or
> journalled-data mode) is to perform a single fsync(), with the
> understanding that this has the side-effect of fsyncing all the other
> files.  That's an ugly solution and is rather hard to do if the workload
> consists of five separate processes!
> 
> So I'd recommending mounting the filesystem with the `-o data=writeback'
> mode.  This way, each fsync(fd) will sync fd's data only.  That's much
> better than the default data-ordered mode, wherein a single fsync() will
> sync all the other file's data too.
> 
> In data=writeback mode it is still the case that fsync(fd) will sync the
> other file's metadata, but that's a single linear write to the journal and
> the additional cost should be low.
> 
> Bottom line: please try data=writeback, let me know.
> 


  Does this mean that those of us using XFS should run some testing as well?


thanks,
   joshua