[dm-devel] dm-raid: add RAID discard support
Mike Snitzer
snitzer at redhat.com
Wed Oct 1 13:32:37 UTC 2014
On Tue, Sep 30 2014 at 10:56pm -0400,
NeilBrown <neilb at suse.de> wrote:
> On Wed, 24 Sep 2014 13:02:28 +0200 Heinz Mauelshagen <heinzm at 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
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