[dm-devel] [PATCH] Disable partitions scan for multipathed devices

[Peter Rajnoha]

On 03/12/2014 01:55 PM, Hannes Reinecke wrote:
> On 03/12/2014 01:46 PM, Peter Rajnoha wrote:
>> On 03/12/2014 01:27 PM, Hannes Reinecke wrote:
>>> On 03/12/2014 01:23 PM, Alasdair G Kergon wrote:
>>>> On Wed, Mar 12, 2014 at 12:48:56PM +0100, Hannes Reinecke wrote:
>>>>> For this I've implemented a new feature 'no_partitions' which
>>>>> just serves as a notifier to kpartx to _not_ create partitions
>>>>> on these devices.
>>>>  
>>>> This should be covered by the existing cookie flags (which udev rules
>>>> already use, if you're using udev).  It's not a multipath-specific
>>>> problem but can apply to other targets too.
>>>>
>>> Ah.
>>>
>>> How? To my knowledge the 'cookies' mechanism is to enable
>>> libdevmapper to wait until udev is done with device creation.
>>>
>>
>> Thing with calling kpartx in udev is that it's only used for mpath
>> devices at the moment, not for all dm devices in general. We could
>> add a hook to call kpartx for each dm device, not just limiting it
>> to the mpath, but there has been no call for this yet...
>>
>> As for the flags encoded in the cookie - the cookie has two parts -
>> the lower 16 bits are used for synchronization identifier, the
>> higher 16 bits are used for passing flags. Out of these 16 'flag bits',
>> 8 bits are common for all DM devices and they're meant to be used for
>> flagging DM devices in general and the rest 8 bits are meant to be used
>> for specific DM subsystem use (see also
>> https://git.fedorahosted.org/cgit/lvm2.git/tree/libdm/libdevmapper.h#n1750)
>>
>> The flags are set with the dm_set_cookie libdevmapper call.
>>
>> The mpath already uses some bits from the "common" group and 1 flag from
>> the subsystem specific group (that is applicable only to mpath). We could
>> check if there's a combination we could reuse, but I think that in this
>> case we should just introduce a new mpath subsystem specific flag that
>> would direct udev rules to skip the kpartx call.
>>
>> Now the question is whether there is anything else we need to skip besides
>> the kpartx call so we cover everything we need for the "host/guest" scenario.
>>
> The only other problem I could think of is the ominous 'blkid' call
> problem.
> 
> Currently 'blkid' is run for every 'change' event,
> And multipath is very keen on generating 'change' events under a
> variety of situations, _plus_ there is the in-kernel
> 'PATH_FAILED/PATH_REINSTATED' mechanism.
> Each of which (except PATH_FAILED) will cause 'blkid' to be run
> by virtue of 13-dm-disk.rules.
> (Actually, someone should expand that to cover 'PATH_REINSTATED', too)
> 
> So when that multipath device happens to have no dependencies left
> or all of those are failed, 'blkid' will hang until the device
> becomes useable again.

This should be covered by latest changes in multipath - I revisited
this a month ago with Ben and we've introduced some new udev rules
to avoid running blkid in case we've run out of paths (there's a new
11-dm-mpath.rules). Though it's not yet upstream, but it has already
been proposed on this list (we did this for RHEL originally to solve
some problems we've been running into).

So this is already resolved, I hope.

-- 
Peter