Dr Data - Davor Vusir tells you how to...

Create a mirrored Flaschache device
root@ohlala:~# mdadm --verbose --create /dev/md1 --level=1 --raid-devices=2 /dev/sdb4 /dev/sdd4
mdadm: Note: this array has metadata at the start and
may not be suitable as a boot device.  If you plan to
store '/boot' on this device please ensure that
your boot-loader understands md/v1.x metadata, or use
--metadata=0.90
mdadm: size set to 40751284K
Continue creating array? y
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md1 started.
root@ohlala:~#

Check the progress.
watch -d cat /proc/mdstat:
Every 2.0s: cat /proc/mdstat                            Sun Apr 29 09:35:03 2012

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md1 : active raid1 sdd4[1] sdb4[0]
40751284 blocks super 1.2 [2/2] [UU]
[>....................]  resync =  0.8% (342656/40751284) finish=7.8min speed=85664K/sec

md0 : active raid1 sdd2[1] sdb2[0]
9764529 blocks super 1.2 [2/2] [UU]

unused devices: <none>

When the synchronization is ready add the array to /etc/mdadm/mdadm.conf by typing:
mdadm --detail --scan >> /etc/mdadm/mdadm.conf

Edit /etc/mdadm/mdadm.conf and put your mailaddress. Also comment out old arrays:
# mdadm.conf
#
# Please refer to mdadm.conf(5) for information about this file.
#

# by default (built-in), scan all partitions (/proc/partitions) and all
# containers for MD superblocks. alternatively, specify devices to scan, using
# wildcards if desired.
#DEVICE partitions containers

# auto-create devices with Debian standard permissions
CREATE owner=root group=disk mode=0660 auto=yes

# automatically tag new arrays as belonging to the local system
HOMEHOST <system>

# instruct the monitoring daemon where to send mail alerts
MAILADDR [email protected]

# definitions of existing MD arrays
#ARRAY /dev/md/0 metadata=1.2 UUID=ae2235f0:110a54e7:6540e885:863f829b name=ohlala:0

# This file was auto-generated on Sun, 29 Apr 2012 08:54:32 +0200
# by mkconf $Id$
ARRAY /dev/md/0 metadata=1.2 name=ohlala:0 UUID=ae2235f0:110a54e7:6540e885:863f829b
ARRAY /dev/md/1 metadata=1.2 name=ohlala:1 UUID=0e8cb4ce:a09d1148:5b289d1b:9ae61d2b

After reboot cat /proc/mdstat. If you get an ouput similar to this below try
mdadm --stop /dev/md127, mdadm --assemble /dev/md1 and update-initramfs -u. Reboot and check again.

md127 : active (auto-read-only) raid1 sdd4[1] sdb4[0]
40751284 blocks super 1.2 [2/2] [UU]

Create the data array
Create new partitions on the data disks. Type is Linux RAID (fd00)

root@ohlala:~# gdisk /dev/sdc
GPT fdisk (gdisk) version 0.8.1

Partition table scan:
MBR: protective
BSD: not present
APM: not present
GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): n
Partition number (1-128, default 1):
First sector (34-3907029134, default = 34) or {+-}size{KMGTP}:
Information: Moved requested sector from 34 to 2048 in
order to align on 2048-sector boundaries.
Use 'l' on the experts' menu to adjust alignment
Last sector (2048-3907029134, default = 3907029134) or {+-}size{KMGTP}:
Current type is 'Linux filesystem'
Hex code or GUID (L to show codes, Enter = 8300): fd00
Changed type of partition to 'Linux RAID'

Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): Y
OK; writing new GUID partition table (GPT).
The operation has completed successfully.
root@ohlala:~#

And create the array: