Use Linux as a SAN Provider

At one-tenth the cost of the typical commercial appliance, Linux can deliver storage with speed and redundancy. Make the move toward a full-featured iSCSI SAN solution with what you already have in your server room.
Setting Up the Initiator and Target

In preparation for setting up the target, you need to provide it with a disk. This can be a physical disk or you can create a disk image. In order to set up a disk image, run the dd command:

dd if=/dev/zero of=/srv/iscsi.image.0 bs=1 seek=10M count=1

This command creates a file about 10MB called /srv/iscsi.image.0 filled with zeros. This is going to represent the first iscsi disk. To create another, do this:

dd if=/dev/zero of=/srv/iscsi.image.1 bs=1 seek=10M count=1

Configuration for the IET software is located in /etc/ietd.conf. Though a lot of tweaks are available in the file, the important lines really are just the target name and LUN. For each target, exported disks must have a unique LUN. Target names are formatted specially. The official term for this name is the iSCSI Qualified Name (IQN).

The format is:

iqn.yyyy-mm.(reversed domain name):label

where iqn is required, yyyy signifies a four-digit year, followed by mm (a two-digit month) and a reversed domain name, such as org.michaelnugent. The label is a user-defined string in order to better identify the target.

Here is an example ietd.conf file using the images created above and a physical disk, sdd:

       IncomingUser michael secretpasswd
       OutgoingUser michael secretpasswd
       Lun 0 Path=/srv/iscsi.images.0,Type=fileio
       Lun 1 Path=/srv/iscsi.images.1,Type=fileio
       Lun 2 Path=/dev/sdd,Type=blockio

The IncomingUser is used during discovery to authenticate iSCSI initiators. If it is not specified, any initiator will be allowed to connect to open a session. The OutgoingUser is used during discovery to authenticate the target to the initiator. For simplicity, I made them the same in this example, but they don't need to be. Note that both of these are required by the RFC to be 12 characters long. The Microsoft initiator enforces this strictly, though the Linux one does not.

Start the server using /etc/init.d/iscsitarget start (this may change depending on your distribution). Running ps ax | grep ietd will show you that the server is running.

Now you can move on to setting up the initiator to receive data from the target. To set up an initiator, place its name (in IQN format) in the /etc/iscsi/initiatorname.iscsi file (or possibly /etc/initiatorname.iscsi). An example of a well-formatted file would be the following:

In addition, you also need to modify the /etc/iscsi/iscsid.conf file to match the user names and passwords set in the ietd.conf file above:

node.session.auth.authmethod = CHAP
node.session.auth.username = michael
node.session.auth.password = secretpasswd
node.session.auth.username_in = michael
node.session.auth.password_in = secretpasswd
discovery.sendtargets.auth.authmethod = CHAP
discovery.sendtargets.auth.username = michael
discovery.sendtargets.auth.password = secretpasswd
discovery.sendtargets.auth.username_in = michael
discovery.sendtargets.auth.password_in = secretpasswd

Once this is done, run the iscsiadm command to discover the target.

iscsiadm -m discovery -t sendtargets -p -P 1

This should output the following:

          IFace Name: default

Now, at any time, you can run:

iscsiadm -m node -P1

which will redisplay the target information.

Now, run /etc/init.d/iscsi restart. Doing so will connect to the new block devices. Run dmesg and fdisk -l to view them. Because these are raw block devices, they look like physical disks to Linux. They'll show up as the next SCSI device, such as /dev/sdb. They still need to be partitioned and formatted to be usable. After this is done, mount them normally and they'll be ready to use.

This sets up the average iSCSI volume. Often though, you may want machines to run entirely diskless. For that, you need to run root on iSCSI as well. This is a bit more involved. The easiest, but more expensive way is to employ a network card with iSCSI built in. That allows the card to mount the volume and present it without having to do any additional work. On the downside, these cards are significantly more expensive than the average network card.

To create a diskless system without an iSCSI-capable network card, you need to employ PXE boot. This requires that a DHCP server be available in order for the initiator to receive an address. That DHCP server will have to refer to a TFTP server in order for the machine to download its kernel and initial ramdisk. That kernel and ramdisk will have iSCSI and discovery information in it. This enables the average PXE-enabled card to act as a more expensive iSCSI-enabled network card.


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