[PATCH 58/71] xfs: garbage collect old cowextsz reservations

[Darrick J. Wong]

On Tue, Sep 27, 2016 at 11:50:21AM -0700, Christoph Hellwig wrote:
> On Mon, Sep 26, 2016 at 02:52:09PM -0700, Darrick J. Wong wrote:
> > The two functions /could/ be merged but I'm hesitant to combine them
> > because they run as different workqueue items.
> > 
> > Setting speculative_cow_prealloc_lifetime to a (much) higher value than
> > speculative_prealloc_lifetime has been useful for combatting CoW
> > fragmentation on VM hosts where the VMs experience bursty write
> > behaviors and we can keep the utilization ratios low enough that we
> > don't start to run out of space.  IOWs, it benefits us to keep the CoW
> > fork reservations around for as long as we can unless we run out of
> > blocks or hit inode reclaim.
> 
> Ok, so there is a good use case for it.  It just felt to me like
> there was a little bit too much duplication, that's why I asked.

I'll add a brief explanation of this to the patch changelog to
record the justification.

--D