Problem: "hardware" (fake) raid broke. I'd like to rebuild the raid using software raid. I brought the system back up on one disk (sdb), and these are the commands I used to configure software raid1

# pvdisplay
  Found duplicate PV nYSwBZCJXIfjgABkZBkBDNYL9fb8Obnq: using /dev/sdb2 not /dev/sda2
  --- Physical volume ---
  PV Name               /dev/sdb2 <-- using sdb2
  VG Name               VolGroup00
  PV Size               74.42 GiB / not usable 14.77 MiB <-- unused space! enough to fit software raid
  Allocatable           yes
  PE Size               32.00 MiB
  Total PE              2381
  Free PE               1
  Allocated PE          2380
  PV UUID               nYSwBZ-CJXI-fjgA-BkZB-kBDN-YL9f-b8Obnq


# dmsetup ls --tree
VolGroup00-LogVol01 (253:1)
 └─ (8:18) <-- yup, /dev/sdb2
VolGroup00-LogVol00 (253:0)
 └─ (8:18)

# fdisk /dev/sda
t
2
fd <-- Changed system type of partition 2 to fd (Linux raid autodetect)
w

# mdadm --create /dev/md1 --level=1 --raid-disks=2 --metadata=0.90 missing /dev/sda2
mdadm: array /dev/md1 started.

# mdadm --examine --scan >/etc/mdadm.conf

# mv /boot/initramfs-`uname -r`.img /boot/initramfs-`uname -r`.img.orig
# mkinitrd /boot/initramfs-`uname -r`.img `uname -r` <-- mdadm.conf is stored on the initramfs, rebuild it

# pvdisplay
  --- Physical volume ---
  PV Name               /dev/md1 <--- got confused, must have a priority for software raid

# dmsetup ls --tree
VolGroup00-LogVol01 (253:1)
 └─ (8:18) <- still using /dev/sdb2
VolGroup00-LogVol00 (253:0)
 └─ (8:18) <- ditto

# pvdisplay /dev/sdb2
  get_pv_from_vg_by_id: vg_read_internal failed to read VG localhost.localdomain

  --- Physical volume ---
  PV Name               /dev/md1 <-- still very confused
  VG Name               VolGroup00
  PV Size               74.42 GiB / not usable 14.77 MiB
  Allocatable           yes
  PE Size               32.00 MiB
  Total PE              2381
  Free PE               1
  Allocated PE          2380
  PV UUID               nYSwBZ-CJXI-fjgA-BkZB-kBDN-YL9f-b8Obnq

# pvcreate -ff /dev/md1
Really INITIALIZE physical volume "/dev/md1" of volume group "VolGroup00" [y/n]? y
  WARNING: Forcing physical volume creation on /dev/md1 of volume group "VolGroup00"
  Physical volume "/dev/md1" successfully created

# pvdisplay
  --- Physical volume ---
  PV Name               /dev/sdb2
  VG Name               VolGroup00
  PV Size               74.42 GiB / not usable 14.77 MiB
  Allocatable           yes
  PE Size               32.00 MiB
  Total PE              2381
  Free PE               1
  Allocated PE          2380
  PV UUID               nYSwBZ-CJXI-fjgA-BkZB-kBDN-YL9f-b8Obnq

  "/dev/md1" is a new physical volume of "74.42 GiB"
  --- NEW Physical volume ---
  PV Name               /dev/md1
  VG Name
  PV Size               74.42 GiB
  Allocatable           NO
  PE Size               0
  Total PE              0
  Free PE               0
  Allocated PE          0
  PV UUID               qkTBV1-Sjuh-NMAa-7HWG-WxkT-Qte2-ChlfTU

# vgextend VolGroup00 /dev/md1
  Volume group "VolGroup00" successfully extended

# pvmove /dev/sdb2 /dev/md1
  /dev/sdb2: Moved: 0.0%
...
  /dev/sdb2: Moved: 100.0%

[during the above]
# dmsetup ls --tree
VolGroup00-LogVol01 (253:1)
 └─VolGroup00-pvmove0 (253:2)
    ├─ (9:1) <-- /dev/md1
    └─ (8:18) <-- /dev/sdb2
VolGroup00-LogVol00 (253:0)
 └─VolGroup00-pvmove0 (253:2)
    ├─ (9:1)
    └─ (8:18)

# vgdisplay | grep PE
  PE Size               32.00 MiB
  Total PE              4762
  Alloc PE / Size       4760 / 148.75 GiB <-- doubled the amount allocated during the move
  Free  PE / Size       2 / 64.00 MiB

[pvmove done]
# pvscan
  PV /dev/sdb2   VG VolGroup00   lvm2 [74.41 GiB / 74.41 GiB free] <-- now unused
  PV /dev/md1    VG VolGroup00   lvm2 [74.41 GiB / 32.00 MiB free]
  Total: 2 [148.81 GiB] / in use: 2 [148.81 GiB] / in no VG: 0 [0   ]

# dmsetup ls --tree
VolGroup00-LogVol01 (253:1)
 └─ (9:1) <-- only using /dev/md1
VolGroup00-LogVol00 (253:0)
 └─ (9:1)

# vgreduce VolGroup00 /dev/sdb2
  Removed "/dev/sdb2" from volume group "VolGroup00"

# fdisk /dev/sdb
t
2
fd <-- Changed system type of partition 2 to fd (Linux raid autodetect)
w

# mdadm -a /dev/md1 /dev/sdb2
mdadm: added /dev/sdb2

# cat /proc/mdstat 
Personalities : [raid1]
md1 : active raid1 sdb2[2] sda2[1]
      78035648 blocks [2/1] [_U]
      [>....................]  recovery =  0.3% (266112/78035648) finish=34.0min speed=38016K/sec