On 04/28/2010 10:37 PM, Neil Brown wrote:
> On Wed, 28 Apr 2010 18:19:45 -0700
> Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
>> Neil Brown wrote:
>>> I'm not sure how this fits with imposing platform constraints.
>>> As platform constraints are closely tied to metadata types, it might be OK
>>> to have a metadata-specific tags (imsm=???) and leave to details to the
>>> metadata handler???
>>>
>>> Dan: help me understand these platform constraints: what is the most complex
>>> constraint that you can think of that you might want to impose?
>>
>> At this point we really only need one constraint: prevent controller
>> spanning. If for example I take an existing imsm member off of ahci and
>> reattach it via a usb-to-sata enclosure mdadm needs a policy to prevent
>> associating that drive with anything on ahci.
>>
>> In a pinch this policy can be disabled, but you wouldn't want to rebuild
>> across usb or any other controller because the option-rom only talks
>> ahci and will mark the drive missing.
>>
>> So something like DOMAIN spanning=imsm, to tell mdadm to follow imsm
>> rules for this domain. Where 'spanning' is policy tag??
>>
>
> Thanks.
>
> So we have two different ideas here.
>
> 1/ A given set of devices (paths) are all attached to the one controller.
> 2/ A given array is not allowed to span controllers
>
> The first statement is somewhat similar to a statement about sparegroups.
> It groups devices together is some way.
>
> The second is a policy statement, and is metadata specific to some extent.
> If I create a native-metadata array using the controller, then adding other
> devices from a different controller is a non-issue. It is only when an
> IMSM array is created that it is an issue (and then - only if the array is to
> be used for boot for for multi-boot).
>
> So the ARRAY line could have "exclusive-group=foo" where 'foo' is a group
> name similar to those used for 'spare-group='
> But that isn't much fun for auto-detect and auto-assembly.
>
> Maybe we want to extend the 'auto' line. It gives policy on a per-metadata
> basis. Maybe:
>
> POLICY auto-assemble +1.x -all
> POLICY same-group imsm
>
> where 'same-group' means that all the devices in an array must be in the
> same spare-group. The 'domain' lines assign spare-groups to devices.
>
> Maybe "same-group" could be "same-$tag" so we could have different tags
> for different metadatas....
>
> Is this working for anyone else, or have I lost the plot??
>
> NeilBrown
I keep going back to the idea of just implement the no-spanning policy
for imsm/ddf as the default with a force-override flag and don't bother
putting it into the config anywhere, it simply is.
--
Doug Ledford <dledford@xxxxxxxxxx>
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http://people.redhat.com/dledford
Infiniband specific RPMs available at
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