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    SubjectRe: [PATCH 0/3] sysfs representation of stacked devices (dm/md)
    On Fri, Feb 17, 2006 at 01:00:17PM -0500, Jun'ichi Nomura wrote:
    > These patches provide common representation of dependencies
    > between stacked devices (dm and md) in sysfs.

    I'm neutral on this change so long as it can be done without
    introducing problems for device-mapper.

    > Though md0, dm-0, dm-1 and sd[a-d] contain same LVM2 meta data,
    > LVM2 should pick up md0 as PV, not dm-0, dm-1 and sdXs.
    > mdadm should build md0 from dm-0 and dm-1, not from sdXs.
    > Similar things will happen on 'mount' and 'fsck' if we use
    > file system labels instead of LVM2.

    I can't speak for the 'mount' code base, but I don't think it'll
    make any significant difference to LVM2 - we'd still have to do
    all the same device scanning as we do now because we have to be
    aware of md devices defined in on-disk metadata regardless of
    whether or not the kernel knows about them at the time the
    command is run.

    > Currently, these relationships are determined by each tool
    > combining information like the existence of md metadata
    > and dm dependency ioctl.

    And attempts to open a device exclusively. That's one check LVM2
    does before running 'pvcreate' on a device.

    > thus we only need to check "holders" directory of the device
    > to decide whether the device is used by dm/md.
    > Also we can walk down the "slaves" directories to collect
    > the devices conposing the given dm/md device.

    For device-mapper devices, 'dmsetup deps' and ls --tree already
    gives you this information reasonably efficiently.

    Would others find the proposal useful for non-dm devices?

    And rather than adding code just to dm and md, would it be better
    to implement it by enhancing bd_claim()?

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