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

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Sep 22, 2017 at 11:38:59AM -0700, Eduardo Valentin wrote:
> Hello Willy,
> 
> On Fri, Sep 22, 2017 at 08:11:22PM +0200, Willy Tarreau wrote:
> > 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 ?
> 
> Yeah, I pondered for several days before opening up this backport
> request exactly because of your concerns above.
> 
> But what made me to decide to share the backport for the community
> consideration is the way I see this. To me, this is *not only* a
> matter of performance boost, but also a real fix. Mainly because the
> systems with v4.9.y become unresponsive while performing fstrim
> because of kernel CPU consumption with the fstrim implementation in
> raid0. Meaning, depending on how this is deployed, how you use your
> fs, and how you deploy your raid, this can be a real problem on
> production systems.
> 
> So, yeah, I hope you consider this not only from a performance
> perspective, but a fix for a real issue.

It might be a "fix", but given that this isn't a regression, the reason
for backporting this is pretty low.  If you want faster I/O, upgrade to
a newer kernel, sounds like a good reason to me (not to mention more
features and more bugfixes as well!) :)

So I would need some big explainations by the developers involved here
as to why I should take it (which is what has caused me to take such
things in the past for other subsystems...)

thanks,

greg k-h