[dm-devel] A better solution than "md_component_detection = 1"

[Lars Marowsky-Bree]

On 2005-02-15T20:17:10, shahid shaikh <shahid.shaikh at patni.com> wrote:

Sorry about this late reply, but I just stumbled across this post.

To recap, the issue is that we want to prevent LVM2/EVMS2 etc from
scanning (and using!) the raw physical devices if those are already
claimed/used by multipath.

LVM2 has a special handling for md_component_detection already; what I
want to propose in the following is a bit more general, and I've
discussed it with Hannes before.

Problem: A general solution for finding out whether another component is
using the device we're looking at.

Proposed solution: Introduce sub-devices into the sysfs tree. (And,
automatically, backlink those to parent devices, but that's just so the
tree can be traversed in any order.)

For example, /sys/block/sda would have a sub/ directory with "dm-0" in
there, pointing at /sys/block/dm-0 (surprise). The
/sys/block/dm-0/parent/sd{a,b} entries would do a very surprising
thing.

This would also allow us to handle partitions in DM better: as all
linear mappings on top of /dev/dm-0 would show up in block/dm-0/sub/ and
vice versa.

md could work the same: block/sys/md0/parent/ would list all the devices
(or partitions) which it consists of.

This very simple(?) change would make a number of things much easier to
figure out.


Caveats:

- This obviously only works if the devices are already active, not if
  the specific user is offline; if LVM2 scans before multipath-tools has
  worked its magic, it won't work. 
  
  (But then, multipath-tools can figure out that there's already someone
  else using the raw physical device and not mess with it! Woho, world
  saved!)

- This ONLY exports very rudimentary hierarchical information; it won't
  tell you what the device is used for or how. However, that information
  alone would be very helpful already.
  

Greg, Alasdair? List?


Sincerely,
    Lars Marowsky-Brée <lmb at suse.de>

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