[dm-devel] dm-multipath: Accept failed paths for multipath maps

[John Utz]

What are the release testing criteria for things like this? From a hand wavy perspective i am sure that the meta-requirement is 'don't break anything'. but how is a new release proven to not break anything?

fer instance, does anybody create dm devices, format them with XFS and then run xfstests against them?

or some other formalized regimen?

or is it all kinda ad-hoc?

tnx!

johnu
________________________________________
From: dm-devel-bounces at redhat.com [dm-devel-bounces at redhat.com] on behalf of Mike Snitzer [snitzer at redhat.com]
Sent: Thursday, July 17, 2014 5:04 PM
To: Stewart, Sean
Cc: device-mapper development; Bryn Reeves; Alasdair Kergon
Subject: Re: [dm-devel] dm-multipath: Accept failed paths for multipath maps

On Wed, Dec 18 2013 at 10:28am -0500,
Stewart, Sean <Sean.Stewart at netapp.com> wrote:

> On Wed, 2013-12-18 at 15:25 +0100, Hannes Reinecke wrote:
> > On 12/18/2013 03:08 PM, Mike Snitzer wrote:
> > > On Wed, Dec 18 2013 at  2:52am -0500,
> > > Hannes Reinecke <hare at suse.de> wrote:
> > >
> > >> The multipath kernel module is rejecting any map with an invalid
> > >> device. However, as the multipathd is processing the events serially
> > >> it will try to push a map with invalid devices if more than one
> > >> device failed at the same time.
> > >> So we can as well accept those maps and make sure to mark the
> > >> paths as down.
> > >
> > > Why is it so desirable to do this?  Reduced latency to restore at least
> > > one valid path when a bunch of paths go down?
> > >
> > Without this patch multipathd cannot update the map as long is
> > hasn't catched up with udev.
> > During that time any scheduling decisions by the kernel part are
> > necessarily wrong, as it has to rely on the old map.
> >
> > > Why can't we just rely on userspace eventually figuring out which paths
> > > are failed and pushing a valid map down?
> > >
> > Oh, you can. This is what we're doing now :-)
> >
> > But it will lead to spurious error during failover when multipathd
> > is trying to push down maps with invalid devices.
> >
> > You are also running into a race window between checking the path in
> > multipathd and pushing down the map; if the device disappears during
> > that time you won't be able to push down the map.
> > If that happens during boot multipathd won't be able to create the
> > map at all, so you might not be able to boot here.
> > With that patch you at least have the device-mapper device, allowing
> > booting to continue.
> >
> > > Are there favorable reports that this new behavior actually helps?
> > > Please quantify how.
> > >
> > NetApp will have; they've been pushing me to forward this patch.
> > Sean?
> >
> Agree.  Internally, we have run into numerous cases with Red Hat where
> the "failed in domap" error will occur, due to user space being behind,
> or device detaching taking too long.  The most severe case is with
> InfiniBand, where the LLD may place a device offline, then every single
> reload that is trying to add a good path in will fail.  I will qualify
> this by saying that I realize it is a problem that the device gets
> placed offline in the first place, but this patch would allow it a
> chance to continue on. The user still has to take manual steps to fix
> the problem in this case, but it seems less disruptive to applications.
>
> The device detaching case could be kind of disruptive to a user in the
> scenario they are upgrading the firmware on a NetApp E-Series box, and
> with this patch, at least a good path is able to be added in ASAP.
>
> > BTW, SUSE / SLES is running happily with this patch for years now.
> > So it can't be at all bad ...
> >
> > Cheers,
> >
> > Hannes
>
> Also agreed.  We have seen this functionality in SLES for years, and
> have not run into a problem with it.

Revisiting this can of worms...

As part of full due-diligence on the approach that SUSE and NetApp have
seemingly enjoyed "for years" I reviewed Hannes' v3 patch, fixed one
issue and did some cleanup.  I then converted over to using a slightly
different approach where-in the DM core becomes a more willing
co-conspirator in this hack by introducing the ability to have
place-holder devices (dm_dev without an opened bdev) referenced in a DM
table.  The work is here:
http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=throwaway-dm-mpath-placeholder-devs

Here is the diffstat of all 3 patches rolled up:

 git diff d4bdac727f1e09412c762f177790a96432738264^..7681ae5ddb5d567800023477be7ddc68f9812a95 | diffstat
 dm-mpath.c |   51 +++++++++++++++++++++++++++++++++++----------------
 dm-table.c |   53 ++++++++++++++++++++++++++++++++++++++++-------------
 dm.c       |    5 ++---
 dm.h       |   12 ++++++++++++
 4 files changed, 89 insertions(+), 32 deletions(-)

But it was only compile tested, because doing more validation of this
work would mean it has a snowballs chance in hell of seeing the light of
upstream.  Sadly it doesn't have a good chance; it would require some
compelling proof:
1) that mpath is bullet-proof no matter how crazy a user got with fake
   place-holder devices in their DM tables (coupled with reinstate_path
   messages, etc)
2) that the storage configs that experienced problems with the current
   DM mpath dm_get_device() failures weren't broken to start with (for
   instance ib srp is apparently fixed now.. but those fixes are still
   working their way into RHEL) -- or put differently: I need _details_
   on the NetApp or other legit storage configs that are still
   experiencing problems without a solution to this problem.

... and even with that proof I'm pretty sure Alasdair will hate this
place-holder approach and will push for some other solution.

I'm going away on paternity leave until Sept 8... my _hope_ is that
someone fixes multipath-tools to suck less or that a more clever
solution to this problem is developed locally in DM mpath.

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