Re: dm-raid: add RAID discard support

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Sep 30 2014 at 10:56pm -0400,
NeilBrown <neilb@suse.de> wrote:

> On Wed, 24 Sep 2014 13:02:28 +0200 Heinz Mauelshagen <heinzm@redhat.com>
> wrote:
> 
> > 
> > Martin,
> > 
> > thanks for the good explanation of the state of the discard union.
> > Do you have an ETA for the 'zeroout, deallocate' ... support you mentioned?
> > 
> > I was planning to have a followup patch for dm-raid supporting a dm-raid 
> > table
> > line argument to prohibit discard passdown.
> > 
> > In lieu of the fuzzy field situation wrt SSD fw and discard_zeroes_data 
> > support
> > related to RAID4/5/6, we need that in upstream together with the initial 
> > patch.
> > 
> > That 'no_discard_passdown' table line can be added to dm-raid RAID4/5/6 
> > table
> > lines to avoid possible data corruption but can be avoided on RAID1/10 
> > table lines,
> > because the latter are not suffering from any  discard_zeroes_data flaw.
> > 
> > 
> > Neil,
> > 
> > are you going to disable discards in RAID4/5/6 shortly
> > or rather go with your bitmap solution?
> 
> Can I just close my eyes and hope it goes away?
> 
> The idea of a bitmap of uninitialised areas is not a short-term solution.
> But I'm not really keen on simply disabling discard for RAID4/5/6 either. It
> would  mean that people with good sensible hardware wouldn't be able to use
> it properly.
> 
> I would really rather that discard_zeroes_data were only set on devices where
> it was actually true.  Then it wouldn't be my problem any more.
> 
> Maybe I could do a loud warning
>   "Not enabling DISCARD on RAID5 because we cannot trust committees.
>    Set "md_mod.willing_to_risk_discard=Y" if your devices reads discarded
>    sectors as zeros"
> 
> and add an appropriate module parameter......

I had the same thought and would be happy with this too.  I was going to
update Heinz's patch to have the same default off but allow user to
enable:
https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=for-next&id=8e0cff64f35971135a6de7907bbc8c3a010aff8f

But I'd love to just follow what you arrive at with MD (using the same
name for the module param in dm-raid).

I'm open to getting this done now and included in 3.18 if you are.

Mike