Re: [dm-devel] [PATCH 0/3] dm raid/raid1: enable discard support when any devices support discard

[Ondrej Kozina]

On 4/19/20 4:48 PM, Paul Wise wrote:
> On Sun, 2020-04-19 at 09:19 -0400, Mike Snitzer wrote:
>
> > You went overboard with implementation before checking to see if your
> > work would be well received.  Your 2/3 patch header shows you're
> > capable of analyzing past commits to explain the evolution of code,
> > etc.  But yet you make no mention of this commit header which explicitly
> > speaks to why what you're proposing is _not_ acceptable:
> >
> > commit 8a74d29d541cd86569139c6f3f44b2d210458071
> > Author: Mike Snitzer <snitzer@xxxxxxxxxx>
> > Date:   Tue Nov 14 15:40:52 2017 -0500
> >
> >      dm: discard support requires all targets in a table support discards
>
> I do remember seeing this commit while working on this, I guess I
> ignored it in my attempts to get fstrim working on my rootfs, woops.
>
> > I haven't looked closely at MD raid in this area but if you trully think
> > underlying MD raid can cope with issuing discards to devices that don't
> > support them (or that it avoids issuing them?) then please update
> > dm-raid.c to conditionally set ti->discard_supported (if not all devices
> > support discard).  That is how to inform DM core that the target knows
> > better and it will manage discards issued to it.  It keeps the change
> > local to dm-raid.c without the flag-day you're proposing.
>
> On my system I have a HDD and an SSD, with /boot on MD RAID and / on
> ext4 on DM RAID on 2 DM crypt volumes. In this setup fstrim works on
> /boot but does not work on / and with my patches it works on / again.
> In addition I don't see any messages in dmesg or other issues when
> doing fstrim / with my patches.

Did you have discard allowed on both dm-crypt devices? dm-crypt (kernel) 
does not allow discards by default.

Regards O.