Re: dm-multipath: Accept failed paths for multipath maps

From: Mike Snitzer <snitzer@redhat.com>
To: Hannes Reinecke <hare@suse.de>
Cc: device-mapper development <dm-devel@redhat.com>,
	Bryn Reeves <breeves@redhat.com>,
	"Stewart, Sean" <Sean.Stewart@netapp.com>,
	Alasdair Kergon <agk@redhat.com>
Subject: Re: dm-multipath: Accept failed paths for multipath maps
Date: Fri, 18 Jul 2014 12:15:50 -0400	[thread overview]
Message-ID: <20140718161550.GB5598@redhat.com> (raw)
In-Reply-To: <20140718160449.GA5598@redhat.com>

On Fri, Jul 18 2014 at 12:04pm -0400,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Fri, Jul 18 2014 at  2:00am -0400,
> Hannes Reinecke <hare@suse.de> wrote:
> 
> > Will be sending some update once the testing is done.
> 
> OK.  The stronger case that can be made for the stability of this change
> (no missing checks for dev->bdev, etc) the better.
> 
> I did just do some basic testing, of both dm-mpath and dm-thinp, seems
> fine.  The DM core changes really are simple (just some extra negative
> checks that won't ever apply for the normal case -- where "normal case"
> is DM targets that use dm_get_device rather than __dm_get_device, and
> had to add a dev_t to struct dm_dev).

BTW, I noticed one thing that I didn't think of as logical fallout, but
it makes sense: if configure multipthd to not use find_multipaths then
multipathd will successfully push down an mpath table that covers the
system disk (sda) -- but mpatha isn't able to dm_get_device:

# lsblk /dev/sda /dev/mapper/mpatha
NAME                            MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
sda                               8:0    0 930.5G  0 disk  
├─sda1                            8:1    0   500M  0 part  /boot
└─sda2                            8:2    0   930G  0 part  
  ├─rhel_rhel--storage--02-swap 253:0    0     4G  0 lvm   [SWAP]
  └─rhel_rhel--storage--02-root 253:1    0   350G  0 lvm   /
mpatha                          253:6    0 930.5G  0 mpath 

# multipath -ll mpatha
mpatha (36003005700ec189015d42b92237a76d1) dm-6 LSI     ,RAID 5/6 SAS 6G 
size=930G features='0' hwhandler='0' wp=rw
`-+- policy='service-time 0' prio=1 status=enabled
  `- 0:2:0:0 sda  8:0   failed ready running

And multipathd keeps failing to reinstate_path:
...
[ 1906.647322] device-mapper: multipath: message: error getting device 8:0
[ 1912.652652] device-mapper: multipath: message: error getting device 8:0
[ 1918.657949] device-mapper: multipath: message: error getting device 8:0
[ 1924.664118] device-mapper: multipath: message: error getting device 8:0
[ 1930.669635] device-mapper: multipath: message: error getting device 8:0

Again, this all makes sense... but certainly shows that blacklisting the
relevant devices is a must.

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