The ISCSI technology is becoming more and more popular over the years as networks (both LAN and WAN) are becoming a lot faster, reliable and robust. Below are some of the advantages to consider when choosing what SAN to go for.
1. The main attraction to a ISCSI box is the tidy price tag that they come with in comparison to your traditional HP/Dell etc branded fibre SAN. If you do some shopping you will soon realise just how cheap they are. You would have change from £1000 for a good ISCSI SAN with some nice storage also. These days SATA hard drives are so cheap and the capacity is rising every year by the 500GB to 1TB region. You can currently buy a Seagate barracuda 2TB for around £80 each, so even a RAID 0+1 array will give you a tidy storage capacity for your network.
2. Location… unlike a fibre or SCSI SAN which has to have a fibre cable or SCSI cable plugged direct from the back of the server to the storage box itself ISCSI can literally be plugged in to any network switch in your infrastructure. Even if it is on a different subnet so long as it has a route back to the server which requires the use for the storage.
3. No HBA overhead. That’s right no need to spec out a server with a fibre HBA or even pay for those expensive fibre cables. ISCSI protocol is encapsulated in the IP packet and the client server which is to have the storage attached will have a small piece of software called an initiator which will read these IP packets to reveal the SCSI signals/language.
Of course it comes with its disadvantages… you will be limited to your network speed. But luckily before long 10GB Ethernet should be as standard which will really give the transfer rate on ISCSI a nice boost. If you are connecting to a ISCSI box in another office over a VPN this again will be a big bottle neck, but of course internet connections are becoming faster and cheaper.
Really you shouldn’t consider ISCSI if you are hosting something like a database or Exchange cluster which requires a lot R/W and bandwidth to the storage box but in small environments it should be sufficient.
Ok so you have your new SCSI DAS storage plugged in to a SCSI HBA attached to your Xen server and you want to use this to store virtual machines. First thing to check is that your storage is detected, this can usually be seen when the server is booting and the HBA will show what is connected/detected.
The next thing we need to do is get on to the Linux console on your Xen Server which can be done using Xen Centre.
Once you are here you will need to use the following command to find the SCSI volume name: ll /dev/disk/by-id/
you will be prompted the following output:
[root@xenserver ~]# ll /dev/disk/by-id/
total 0
lrwxrwxrwx 1 root root 16 Jul 9 16:09 cciss-3600508b1001030333520202020200000 -> ../../cciss/c0d0
lrwxrwxrwx 1 root root 18 Jul 9 16:09 cciss-3600508b1001030333520202020200000-part1 -> ../../cciss/c0d0p1
lrwxrwxrwx 1 root root 18 Jul 9 16:09 cciss-3600508b1001030333520202020200000-part2 -> ../../cciss/c0d0p2
lrwxrwxrwx 1 root root 18 Jul 9 16:09 cciss-3600508b1001030333520202020200000-part3 -> ../../cciss/c0d0p3
lrwxrwxrwx 1 root root 9 Jul 9 16:09 scsi-SPromise_3_Disk_RAID5_58544_8_55010000 -> ../../sda
The part we are interested in is the “scsi-SPromise_3_Disk_RAID5_58544_8_55010000“. Copy and save this to notepad for now.
Next we need to add a LVM storage repositry using the following command:
xe sr-create type=lvm content-type=user device-config:device=/dev/disk/by-id/scsi-Volume-Lable name-label=”LOCAL SR”
Where “SCSI-Volume-Lable” is in our example scsi-SPromise_3_Disk_RAID5_58544_8_55010000 and the “LOCAL SR” is any name you want to have displayed for the SR in XenServer.
Below is an example of the full command:
xe sr-create type=lvm content-type=user device-config:device=/dev/disk/by-id/scsi-SPromise_3_Disk_RAID5_58544_8_55010000 name-label=”Xen SCSI Storage”
Once this is done you will see your new storage in the left where you can start storing VM’s.
We recently visited a customer’s site that required a hosted Exchange environment. Upon visiting the client we found that they a spare old server which used to be an old Domino server which had 1TB of disk space.
We decided to use this spare server and install an open source iSCSI SAN software called Openfiler. Essentially this is a Linux based OS that you will install on a server (or even a virtual server), then use the web interface to setup all your LUNs and partitions.This free software worked great for the clustered Exchange solution and the client saved around £1500.00 instead of paying for a iSCSI SAN.