add vdev_id support to zfs-initramfs

[nedbass]

Before we deprecate zpool_id we need to provide a migration path for root-on-ZFS users. To that end vdev_id must be intetgrated with zfs-initramfs so that a root pool using /dev/disk/by-vdev names will be properly imported.
#981

[ryao]

Is zfs-initramfs part of ZFSOnLinux upstream? I thought that this was handled at the distribution level.

[nedbass]

You're right, zfs-initramfs is not part of ZOL. I just wanted to make sure this is coordinated with downstream maintainers before we remove zpool_id.

[dajhorn]

@nedbass, pkg-zfs got this change from #811 and it was published as 0.6.0.83.

[ryao]

@nedbass Thanks for your concern. Gentoo's genkernel software uses mdev, so none of the functionality that depends upon udev helpers currently works. The gentoo bug tracker has no requests for it. Removing zpool_id will not cause any regressions in downstream Gentoo.

[nedbass]

@dajhorn It looks like pkg-zfs integrated support for zpool_id but not for vdev_id. Per #981 we are interested in retiring zpool_id in favor of vdev_id.

[dajhorn]

@nedbass, okay, right. I will add 69-vdev.rules and vdev_id to the manifest.

[behlendorf]

Thanks @nedbass @ryao @dajhorn. Since we'll be dropping the zpool_id bits in -rc14 we'll want to make sure the FAQ gets updated accordingly so people using the legacy scripts can easily migrate to the improved vdev_id scripts.

[behlendorf]

Closing, I've just retired zpool_id/zpool_layout and @dajhorn has updated the initramfs.

dbf763b Retire zpool_id infrastructure
dajhorn/pkg-zfs@bccc23f Add the vdev_id udev helper to the initramfs hook.