Fedora Linux

Block size=1024 (log=0)
Fragment size=1024 (log=0)
32768 inodes, 130944 blocks
6547 blocks (5.00%) reserved for the super user
First data block=1
Maximum filesystem blocks=67371008
16 block groups
8192 blocks per group, 8192 fragments per group
2048 inodes per group
Superblock backups stored on blocks: 8193, 24577, 40961, 57345, 73729
Writing inode tables: done
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 27 mounts or 180 days, whichever comes first. Use tune2fs -c or -i to override.
# mkdir /mnt/database
# mount /dev/md0 /mnt/database

Any data you write to /mnt/database will be written to both the local volume and the remote drive.

Do not use iSCSI directly over the Internet: route iSCSI traffic through a private TCP/IP network or a virtual private network (VPN) to maintain the privacy of your stored data. 

To shut down the remote mirror, reverse the steps:

# umount /mnt/database
# mdadm --stop /dev/md0
# iscsiadm -m node --record f68ace --logout

A connection will be made to the remote node whenever the iSCSI daemon starts. To prevent this, edit the file /etc/iscsid.conf :

#
# Open-iSCSI default configuration.
# Could be located at /etc/iscsid.conf or ~/.iscsid.conf
#
node.active_cnx = 1
node.startup = automatic
#node.session.auth.username = dima
#node.session.auth.password = aloha
node.session.timeo.replacement_timeout = 0
node.session.err_timeo.abort_timeout = 10
node.session.err_timeo.reset_timeout = 30
node.session.iscsi.InitialR2T = No
node.session.iscsi.ImmediateData = Yes
node.session.iscsi.FirstBurstLength = 262144
node.session.iscsi.MaxBurstLength = 16776192
node.session.iscsi.DefaultTime2Wait = 0
node.session.iscsi.DefaultTime2Retain = 0
node.session.iscsi.MaxConnections = 0
node.cnx[0].iscsi.HeaderDigest = None
node.cnx[0].iscsi.DataDigest = None
node.cnx[0].iscsi.MaxRecvDataSegmentLength = 65536
#discovery.sendtargets.auth.authmethod = CHAP
#discovery.sendtargets.auth.username = dima
#discovery.sendtargets.auth.password = aloha

Change the node.startup line to read:

node.startup = manual

Once the remote mirror has been configured, you can create a simple script file with the setup commands:

#!/bin/bash
iscsiadm -m node --record f68ace --login
mdadm --assemble /dev/md0 /dev/main/database /dev/sdi1
mount /dev/md0 /mnt/database

And another script file with the shutdown commands:

#!/bin/bash
umount /mnt/database
mdadm --stop /dev/md0
iscsiadm -m node --record f68ace --logout

Save these scripts into /usr/local/sbin and enable read and execute permission for both of them:

# chmod u+rx /usr/local/sbin/ remote-mirror-start
# chmod u+rx /usr/local/sbin/ remote-mirror- stop

You can also install these as init scripts (see Lab 4.6, 'Managing and Configuring Services and Lab 4.12, 'Writing Simple Scripts ').

6.2.3.4. ...using more than one RAID array, but configuring one hot spare to be shared between them?

This can be done through /etc/mdadm.conf . In each ARRAY line, add a spare- group option: