join_superset and isw.c

Phillip Susi psusi at
Mon Nov 10 18:28:44 UTC 2008

I've been trying to fix a bug in Ubuntu's dmraid package based on rc14 
and using a patch from mandriva to add raid10 support to isw.c, and I 
have noticed some odd things that I thought I would ask about here.

I noticed that most other ataraid format handlers use join_superset() in 
format.c, which I believe is used to synchronize things like the devs 
member in the different raid set structures that get created to 
represent the various components of the overall raid set.  The mandriva 
patch modifies isw.c to also make use of join_superset, and in doing so, 
changed name() to have the same signature as the other format handlers, 
which is to say, it removed the struct isw_dev parameter.  I now see the 
reason it did this was because join_superset is given a pointer to this 
function so the function needs to match the signature that join_superset 
expects.  The problem this caused was that name() always went with the 
first isw_dev found, so a raid set with multiple volumes defined in it 
would end up failing to activate because the devices got added to the 
first volume twice instead of once to each of the two different volumes.

The strange thing is that rc15 appears to have raid10 support built in, 
but it does not make use of join_superset().  This leaves me wondering a 
few things:

1) Do the other formats support multiple volumes defined within the raid 
set, and if so, how do they do so without passing a parameter to name() 
identifying the volume in question?

2) Why does isw.c in rc15 not make use of join_superset?  Is it because 
it MUST pass extra parameters to name()?

3) Is my understanding of the structure correct?  It looks like you are 
supposed to end up with one raid_set that describes the family as a 
whole, one that describes each of two raid0 components, one that 
describes the raid1 component, and then one for each of the volumes. 
What I'm not sure of is what raid_devs should be linked to by each of 
those raid_sets.

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