Self-proclaimed genius, and ruler of the Internet. System admin the rest of the time

iSCSI is a protocol that allows you to use a disk in a remote machine, locally as a block device. It is a very popular SAN protocol, which allows the consolidation of storage into one large storage pool.

In iSCSI terminology, a target is a storage resource located on an iSCSI server and an initiator is a client which will be connecting to a target.

In this post I will demonstrate how to setup an iSCSI target and initiator on RedHat Enterprise Linux 5.

Target

RHEL5 has the stgt target daemon included with it. This is the server which allows initiators (clients) to connect to the disks using the iSCSI protocol. It is installable using yum:

yum install scsi-target-utils

To share a disk over iSCSI, you can either use a disk image, or a block device. I prefer to use LVM logical volumes as LVM allows easy management of free space.

Stgt’s configuration file lives in /etc/tgt/targets.conf, although most configuration can be done using the command line (although this is not persistant across reboots, so I prefer to use the configuration file).

To create a target, just add the following lines:

backing-store /dev/DiskArray/Archive1

Please note that you must replace iqn.2009-02.com.hamzahkhan:archive1 with your own domain name, and resource name. It must be in the iSCSI Qualified Name (IQN) format, iqn.yyyy-mm.{reversed domain name}:an_easy_to_remember_lablel.

The second line, backing-store /dev/DiskArray/Archive1 specifies the disk/disk image that is to become the target, I am using LVM logical volume although the procedure is exactly the same for disk images, and real physical disks.

By default, stgt will allow all IPs to connect to the target, which is highly insecure! To change this behaviour, it is possible to specify IP addresses which are allowed to use the targets. To do this, just place the following line under backing-store:

initiator-address 10.1.0.4

This will allow 10.1.0.4 to access the target.

Now, use chkconfig to make stgt start at boot, and start up the daemon.

/etc/init.d/tgtd start

chkconfig tgtd on

Thats all there is to it on the server side! All done

You can now connect to the target using any initiator, such as the one built into Windows Vista (although I have never tried using Windows).

Initiator

RedHat have included an iSCSI daemon which is also installable using yum:

yum install iscsi-initiator-utils

To connect to the target, edit /etc/iscsi/initiatorname.iscsi and change InitiatorName to something you prefer (Remember! it must be in the IQN format, iqn.yyyy-mm.{reversed domain name}:an_easy_to_remember_lablel. I usually use iqn.2009-02.com.hamzahkhan:hostname_of_box). Next start up iSCSId:

/etc/init.d/iscsid start

and use iSCSI target descovery to find the targets on the server:

iscsiadm -m discovery -t st -p $SERVERS_IP

If all is well, it should output the names of all the targets that the initiator is allowed to connect to!

Next, we need to create the disk nodes. To do this, RedHat have provided a nice start up script. This script will login to all the targets that the iSCSI daemon knows about. We have already used the iscsiadm command to tell the iSCSI daemon which targets exist on the server, so using the script is all that is left:

/etc/init.d/iscsi start

Thats all there is to it!

You should have a new disk node in /dev/. You can use lsscsi (yum install lsscsi) to find the exact name if you have a lot of USB/SATA/SCSI drives connected to the machine already.

Now all you have to do is partition the disk, and dump your files onto it

Please remember, you must NEVER mount a partition on two machines at the same time. Doing so will cause data loss!

It IS possible to mount the same disk on multiple machines, but this requires a special clustered filesystem such as GFS.

After three or four quite useless posts, I thought it might be time for a “real” post, so here it goes!

As everyone might know, Facebook have rolled out the new layout and have removed the old one. In a final “fight” to use the old one, A LOT of facebook users joined the Facebook developers group, only to find that the old layout has been removed completely (as far as I know).

A lot of people think that there was not any “real” problem with the old facebook but the issue with the old layout that triggered the need for a new layout was the fact applications were shown on one page. This meant that some pages became VERY cluttered for the users who seemed to add every application available!

In my opinion, the main reason Facebook is such a sucessful site compared to other social networking sites is that it is neat. If you look at Hi5, MySpace, BeBo, they all use quite flat pages (very little to no AJAX!) which the user can modify the layout of through the use of CSS.

Most normal users would tell you they would LOVE to be able to customise the look of their profiles on Facebook, but most geeks would probably cringe at this idea. Why? Most people who use such social networking sites don’t really know much CSS or HTML, so they write bad CSS and create horrid pages which take way too long to render and sometimes even crash the browser.

Users who add too many applications are in effect doing the same thing. There are too many things to render (images, text, colours, sometimes even flash!), and so your over all Facebook “experience” is brought down to something which is not so plesurable.

Although most people may be thinking that the new layout gives them a worse experience, I’m pretty certain, it is much better than having cluttered profile pages which sometimes crash your browser .

I will accept that the new layout COULD be better, but there is no doubt about it, the new layout beats the old one overall.

As some of you might know, I recently switched from VirginMedia (Cable) to O2 Broadband (ADSL).

To do this, I had to get my BT Telephone line enabled, and my VirginMedia Telephone line canceled.

Although this seemed like an easy task, it was actually more difficult than I had expected.

I wanted to keep my old telephone number that I was using on my VirginMedia line, so BT had to do some number porting things which they said should take 1 week or so. So I waited, and after 1 week, it had not been done! In the end I had to wait 1 month for it to be done.

When the number had finally been ported, there was something strange up with the actual line, and I couldn’t get a dial tone, so a BT engineer had to come around and check it out. He fixed it, but he noticed that there was a lot of noise on the line (crackling sounds). After a bit of tweeking, the noise was bareable, but I wasn’t really fully pleased with that, as it would cause a problem with my ADSL connection once it was activated. Funnily, the noise isn’t there all the time, it is there very faintly at some times of the day, and VERY heavy at other times.

Anyway, I called up O2 and ordered the connection, and after a few days my line was activated and ready to go! BUT, I soon found out that everything was not as good as it seemed.

Whenever the phone rings, or the noise is high on the telephone line, my internet connection dropped and the modem had to re-sync, which takes a while. This sucks since I am an IRC user and I also host an IRC server. Every time the connection dropped, I pingout from all the servers I am connected to, and so does everyone who is connected to mine.

So after a bit of investigation (testing in the master test socket, changing filters, trying a different phone etc), I came to the conclusion that the noise on the line is the only thing that could be causing the disconnections. So I called up BT, and they sent an engineer to check it out, BUT the noise was VERY faint at that time, so he had to leave without doing much, (he checked all the connections where he could etc, but understandably that was all he could really do if the noise wasn’t there at the time).

Even after his checks, the noise is still currently there, so I am having to call BT up again. They told me they will send another engineer around who will check it out and hopefully the noise will be there when he gets here. I don’t know how long this will keep going on for, (an engineer coming to check it out, then leaving cause its not there at that time etc).

Hopefully this problem will be resolved soon as it is driving me insane!

Until then, if anyone has any ideas that may help, please comment!