Re: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback_mb

[Dave Chinner]

On Tue, Sep 08, 2009 at 06:56:23PM +0200, Peter Zijlstra wrote:
> On Tue, 2009-09-08 at 12:29 -0400, Chris Mason wrote:
> > Either way, if pdflush or the bdi thread or whoever ends up switching to
> > another file during a big streaming write, the end result is that we
> > fragment.  We may fragment the file (ext4) or we may fragment the
> > writeback (xfs), but the end result isn't good.
> 
> OK, so what we want is for a way to re-enter the whole
> writeback_inodes() path onto the same file, right?

No, that would take use back to the Bad Old Days where one large
file write can starve out the other 10,000 small files that need to
be written. The old writeback code used to end up in this way
because it didn't rotate large files to the back of the dirty inode
queue once wbc->nr_to_write was exhausted. This could cause files
not to be written back for tens of minutes....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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