Re: [stable v4.9.y] Backports to fix fstrim time / CPU load on raid0

[Willy Tarreau <w@1wt.eu>]

On Fri, Sep 22, 2017 at 10:40:48AM -0700, Eduardo Valentin wrote:
> On Fri, Sep 22, 2017 at 07:31:45PM +0200, Greg KH wrote:
> > On Fri, Sep 22, 2017 at 10:16:06AM -0700, Eduardo Valentin wrote:
> > > Hello GregKH,
> > > 
> > > I have been seeing several reports of performance issue with raid0 while performing fstrim on v4.9.y.
> > > Currently, if one performs:
> > > 
> > > # fio --name fio_test_file --direct=1 --rw=randwrite --bs=4k --size=5G --numjobs=8 --group_reporting --directory=/mount/raid0 
> > > # rm -rf /media/nvme-raid0 
> > > # time fstrim -vvv -a 
> > > real	3m41.102s 
> > > user	0m0.000s 
> > > sys	0m4.964s 
> > 
> > Also, is this a regression from older kernels?
> 
> I personally did not try older than 4.9 kernels. but looking at git history,
> and the commit message of the fix, looks like this is long known issue that
> got fixed on v4.12.

Or it may simply be something that couldn't be achieved without significantly
improving the underlying infrastructure using the patches you've spotted (and
possibly a few others that you didn't notice but are required for stability
or to avoid breaking other subsystems).

While I use 4.9 on many of my machines, I'd rather favor maximal stability
over an optimization for some operations that don't appear *that* often.
That said if the relevant maintainers consider it safe to backport, I'll
certainly welcome some performance improvements on my machines, but that's
not what I'm primarily looking for in stable kernels.

And if we start to backport performance improvements into LTS kernels,
what will encourage users to upgrade to the next LTS ?

Cheers,
Willy