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Subject[RFC]partitions through device-mapper
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Hello, 

now that the device mapper is merged into mainline, I would like to open a
discussion on the possible in-kernel partition handling clean-up.

In-kernel partition handling covers :
o parsing of the on-disk partition tables
o partition block devices creation / structs

Along with initramfs will come the possibility to rip off the partition tables
parsing from the kernel : a userspace parser like partx (part of util-linux
toolset) can teach the kernel the partition layout.

As driverfs provides elegantly block device add/remove events to hotplug, calls
to partx can be wrapped into the block.agent

The device-mapper merging could enable the ripping of all kernel partition
understanding by creating linear device-maps over partitions.

As a proof of concept, I've mutated partx to create those mappings. This tool
is available for testing and commenting at :
http://dsit.free.fr/dmpartx.tar.bz2

This tool cannot damage your data : BLKPG_DEL_PARTITION and
BLKPG_ADD_PARTITION ioctls are removed from the source.

I would like to receive feedback over the following points :

* Is this proposal completely out of the point ? Have I overlooked some
important implementation details ?

* driverfs send block device-removal-event at the end of the job, but device
removal cannot happen as there are device-mappings over it. This ordering
forbids hotplug to trigger the partition-mappings flush before block-device
removal. Can this ordering be changed, or is there another solution ?

* 2.5.44-ac4 did not notify block-device-add upon scsi disk insertion with
scsi-add-single-device, which I think is known by scsi subsystem maintainers.
-> The block.agent provided with dmpartx is not tested.

* Should dmpartx create the 0-lengh partitions ? It does not at the moment.
Extended partitions are handled the same way as partx : resize to 63 blocks


regards,
cvaroqui
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