If you have any questions/corrections/contributions about this how-to, you're welcome on this OpenELEC forum topic

Playing with iSCSI requires some basic knowledge on UNIX/Linux systems and networks. If you have never heard of iSCSI before, or don't really know what it is, chances are that you don't really want to use it.

This part is really dependent on your iSCSI target device, so it can't be covered in this HowTo

The LUN that you create will be used to store the OpenELEC System and Storage partitions, so size it as you want/need (4GB should be enough for starting, and there are chance that your iSCSI system allows you to resize your LUNs later – that's one good reason to use iSCSI, but you can give your system more or less space if you want). Information That You Will Need

Depending on your iSCSI system, you could have to add authorizations (or LUN masquerading) for your initiators (your building system and your HTPC).

So, your LUN is created, now you have to access it from your building system. First you need to have iscsi support

aptitude install open-iscsi

Now you have to discover your iscsi portal

iscsiadm -m discovery -t st -p 192.168.1.14

That should list the available targets. In my example

192.168.1.14:3260,0 iqn.2000-01.com.synology:pandora.openelec

It should have created a config file in /etc/iscsi/nodes/iqn.2000-01.com.synology\:pandora.openelec/192.168.1.14\,3260\,0/default. If you have configured CHAP authentication (and reverse CHAP), you should edit this file (adapt the path to your needs) and add

node.session.auth.authmethod = CHAP
node.session.auth.username = username
node.session.auth.password = password
node.session.auth.username_in = username_in
node.session.auth.password_in = password_in

And finally, login into the target

iscsiadm -m node -T iqn.2000-01.com.synology:pandora.openelec -p 192.168.1.14:3260 -l

If you have some errors like

Logging in to [iface: default, target: iqn.2000-01.com.synology:pandora.openelec, portal: 192.168.1.14,3260]
iscsiadm: Could not login to [iface: default, target: iqn.2000-01.com.synology:pandora.openelec, portal: 192.168.1.14,3260].
iscsiadm: initiator reported error (19 - encountered non-retryable iSCSI login failure)

Then, you should review your authentication configuration. If everything went fine, you should see some logs on the terminal

Logging in to [iface: default, target: iqn.2000-01.com.synology:pandora.openelec, portal: 192.168.1.14,3260]>
Login to [iface: default, target: iqn.2000-01.com.synology:pandora.openelec, portal: 192.168.1.14,3260] successful.

Cool, now you have a new block device on your system.

dmesg | tail -n 8

[ 9648.232779] scsi4 : iSCSI Initiator over TCP/IP
[ 9649.246417] scsi 4:0:0:0: Direct-Access SYNOLOGY iSCSI Storage    3.1  PQ: 0 ANSI: 5
[ 9649.247062] sd 4:0:0:0: [sdc] 16777216 512-byte logical blocks: (8.58 GB/8.00 GiB)
[ 9649.248907] sd 4:0:0:0: [sdc] Write Protect is off
[ 9649.248915] sd 4:0:0:0: [sdc] Mode Sense: 3b 00 00 00
[ 9649.249371] sd 4:0:0:0: [sdc] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
[ 9649.251819] sdc: sdc1 sdc2
[ 9649.253767] sd 4:0:0:0: [sdc] Attached SCSI disk

So for me it's /dev/sdc (there are already partitions on it, with a brand new LUN, that should of course not be the case). Remember what device it is for the next step (I will use /dev/sdX to refer to it, so people who does copy/paste will not destroy any existing data in the next steps).

First you have to create a partition table on your device. You can use your preferred tool for that (fdisk, cfdisk, parted, …). My favourite tool is cfdisk

cfdisk /dev/sdX

Check that the partition table is fine

fdisk -l /dev/sdX

Disk /dev/sdX: 8589 MB, 8589934592 bytes
133 heads, 62 sectors/track, 2034 cylinders, total 16777216 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000eece8

  Device Boot      Start         End      Blocks   Id  System
/dev/sdX1   *         62      503005      251472   83  Linux
/dev/sdX2          503006    16777215     8137105   83  Linux

Now you have to create the two filesystems, System and Storage (the labels are important to have OpenELEC booting fine, if you want/need to change them, you'll have to adapt the extlinux.conf file later)

mkfs.ext4 -L System /dev/sdX1
mkfs.ext4 -L Storage /dev/sdX2

Mount the System partition on your system

mkdir -p /mnt/openelec/system
mount LABEL=System /mnt/openelec/system

Copy the OpenELEC KERNEL and SYSTEM files in it :

cp /path/to/SYSTEM /mnt/openelec/system
cp /path/to/KERNEL /mnt/openelec/system

First, create the extlinux.conf file.

nano /mnt/openelec/system/extlinux.conf

 DEFAULT linux
 PROMPT 0

 LABEL linux
   KERNEL /KERNEL
   APPEND ip=dhcp boot=ISCSI='''ISCSI_OPTIONS''',LABEL=System disk=LABEL=Storage quiet

The ISCSI_OPTIONS will depend on how you will boot. You can edit it later. Basically, there will be two options :

If you are a sane person, you probably already know that you will use an iBFT capable ROM like IPXE

You don't have to specify the ISCSI_OPTIONS for every device, even if you both have System and Storage on iSCSI. Just use it on the boot definition, as in the example Now install extlinux on the device

Now install extlinux on the device

extlinux --install /mnt/openelec/system

Now unmount the device

umount /mnt/openelec/system

Now write the mbr (or gptmbr)

dd if=/usr/share/syslinux/mbr.bin of=/dev/sdx bs=440 count=1

And log out from the target

iscsiadm -m node -T iqn.2000-01.com.synology:pandora.openelec -p 192.168.1.14:3260 -u

There are chances that your target will allow you to access to your device from several computers at the same time. YOU SHOULD NEVER MOUNT IT FROM MORE THAN ONE COMPUTER AT A TIME. Writing on a filesystem from more than one computer at the same time will corrupt your filesystem, you don't want to do that. NEVER !

Ok, so now you have your LUN and OpenELEC is installed on it, but how will you make your HTPC boot from it.

There is several answers to that question.

If you have the chance to have an HTPC which have an iSCSI compatible network adapter (most intel network adapters) it will be really easy just go into your network card BIOS/ROM, and configure the iSCSI target, and tell it to boot from it. You should be done!

You probably don't have such a network adapter (it's more an entreprise-class feature), but no problem, IPXE (http://ipxe.org) is there for you!

IPXE is an OpenSource Boot Firmware, it's a maintained fork of Etherboot/GPXE. It does PXE, is capable to access files from TFTP, FTP, HTTP servers, and more important it is also capable to boot from iSCSI (it also supports san boot on AoE and FCoE) !

Installing a DHCP and/or a TFTP server is out of the scope of this howto. You'll find good tutorials about that on the Net. There are some good docs on the IPXE homepage about that on http://ipxe.org/howto/dhcpd and http://ipxe.org/howto/msdhcp.

If you don't already have a DHCP server nor a TFTP server, I recommend you to look at dnsmasq, which is a lightweight DHCP/TFTP server.

So basically, you just have to make the ipxe ROM available on your TFTP server, and configure the DHCP server to make your box boot on it (next-server...). Then, configure a DHCP chainloader (http://ipxe.org/howto/chainloading) to boot a script like

echo Ready for iscsi boot !
set username username
set password password
set reverse-username username_in
set reverse-password password_in
:retry_sanboot
sanboot iscsi:192.168.1.14::::iqn.2000-01.com.synology:pandora.openelec || goto retry_sanboot

Of course you will have to adapt it to your needs (ip address, target name, usernames and passwords).

You don't want to (or you can't) install and configure a DHCP server and a TFTP server.

And you have that old SD card of 128MB that you earned with your first digital camera years ago, that is of no use now... great! It's a perfect candidate for becoming a booting ROM device! (of course, if your HTPC doesn't allow you to boot from SD card, you'll need to find something else, like an old USB key (the smaller, the better).

Create a new text file, that will be your ipxe embeded boot script

#!ipxe
:retry_dhcp
dhcp || goto retry_dhcp
echo Ready for iscsi boot !
set username username
set password password
set reverse-username username_in
set reverse-password password_in
:retry_sanboot
sanboot iscsi:192.168.1.14::::iqn.2000-01.com.synology:pandora.openelec || goto retry_sanboot

Once again, adapt the values to your needs!