Storage over IP: What is it good for?

We've been hearing for a long time about running storage over IP, particularly using the iSCSI protocol. We take a look at the technology, its applications, and some of the devices you can use.



We've been hearing for a long time about running storage over IP, particularly using the iSCSI protocol. We take a look at the technology, its applications, and some of the devices you can use.

For a long time now expanding storage meant buying more servers and disks. This has proved to be a costly method for increasing storage, and is often an inefficient way of doing things.

We are now instead using storage area networks (SAN). This term generally relates to a bank of storage devices for multiple servers and/or networks that may be accessed in a local area network (LAN) or wide are network (WAN).

The key benefits here are: increased disk utilisation, improved data availability, improved performance and protection of data. SANs also reduce the loads that are placed on servers.

Fibre channel (FC) has been the main way of interconnecting servers and storage devices in SANs for some time now. However, it's very expensive. IP storage, or in particular iSCSI, is a fairly new technology offering lower costs and easier deployment than fibre as iSCSI uses standard Ethernet technology to create storage area networks and greater connectivity.

If you want to get more technical, iSCSI or Internet SCSI is an IETF standard that maps SCSI blocks into Ethernet packets. The iSCSI protocol is a method for transporting low latency SCSI blocks across IP networks.

There are a few other terms we should clear up before we go on. You may have come across the terms Direct Attached Storage (DAS) and Network Attached Storage (NAS).

The differences here are quite simple. DAS units allow system administrators to connect them directly to a server typically via SCSI -- some DAS arrays actually allow multiple servers to be connected directly to them and share their resources, and also provide server redundancy too if required.

NAS are specialised file servers for Ethernet-based networks. NAS boxes allow administrators to connect them somewhere on the local area or wide area network for remote access. By simply attaching a DAS or NAS device directly to a server or on your network you can instantly increase your storage capacity by 1TB or more.

  • Storage over IP
  • Deploying iSCSI
    There are three obvious ways you can deploy iSCSI. The most obvious way is natively, which basically is an all iSCSI SAN where all the devices have an Ethernet interface and connect to an Ethernet LAN and all use the iSCSI protocol without using any sort of translation or specialised equipment. This is the best way a small to mid-sized company could get into SANs.

    Then there is bridging which is when you join Ethernet-based devices that are using the iSCSI protocol to an existing FC SAN. This deployment is typically carried out by organisations that have existing FC SANs and would like to migrate to an Ethernet based SAN. Specialised equipment has to be used here. We talk more about this later on.

    The third way of using iSCSI, extension, is essentially linking SANs across large distances.

    iSCSI considerations:
    iSCSI is limited to current one gigabit Ethernet speeds while fibre channel can run at 2Gbps with 4Gbps just recently being made available. 10Gbps fibre is on the horizon, but so is 10Gbps Ethernet. For the time being, if you have already invested heavily in fibre think carefully before making an investment in iSCSI. At the moment the technology is slower, and over larger distances the delays will be even greater.

    We know that FC works and it would be a safe bet to run mission critical systems over fibre. For now, we can't actually say this about iSCSI.

    iSCSI vendors in our minds are having some doubts over large implementations, particularly because of the large delays that can occur. When 10Gbps Ethernet becomes easily available, we expect to see deployments of iSCSI becoming more common.

    The Storage Network Industry Association (SNIA), who announced the iSCSI standard a few years back, says small to medium businesses will welcome this technology as a means to maximise their return on investment. Despite the big hype vendors have been somewhat slow in certifying their products. We are seeing this technology moving slowly but it is being used successfully by those who are keen to take advantage.

    iSCSI Cards, TOEs and Standard NICs
    You are probably wondering how your servers will connect using iSCSI. There are three ways: with a standard network interface card (NIC) using an iSCSI driver, with a TCP offload engine (TOE) NIC with an iSCSI driver, or by with an host bus adaptor (HBA) designed for iSCSI.

    iSCSI places quite a large overhead on server CPUs. As iSCSI wraps additional information over standard SCSI commands, it creates a huge amount of TCP/IP work which a standard NIC offloads to the CPU. TOEs or HBAs can look after the overheads associated with iSCSI without offloading any work to the main CPU. The main difference between these two types of cards is iSCSI HBAs only handle iSCSI traffic whereas TOE cards handle all Ethernet traffic.

    What will it cost?
    Traditional SCSI DAS deployments have the lowest acquisition costs, however they bear much higher long-term costs, or TCO, than FC SAN and iSCSI SAN. Furthermore, FC SANs have a higher upfront cost that iSCSI SANs.

    For this technology overview we take a first hand look at the Snap Appliance Server 4500 and how it implements iSCSI. We also look at some products from Adaptec, HP, and Cisco.

  • Deploying iSCSI
  • Snap Appliance Server 4500
    Snap Appliance Server 4500
    The Snap Appliance Server 4500 is a network attached storage (NAS) device. It comes in a one rack unit (1RU) chassis and is marginally deeper than it's wide. On the back of the unit is a serial port, video port, a pair of PS/2 connectors, a pair of USB ports, and a pair of gigabit Ethernet ports. There was also an Ultra SCSI 160 port that is provided by an internal card.

    Inside was a 2.4GHz Intel Pentium 4 processor with 512MB of RAM and provision for two extra memory modules. To gain access to the hard disks you have to remove the front facia then pull out the drive cradles. There were four drive cradles in total. In these cradles were 250Gb Western Digital drives to give you a combined storage capacity of 1TB.

    The Snap Server doesn't take long to install and configure. All we did was use a network cable to connect it up to one of our switches. From there we installed the Snap Server Manager on a Windows 2003 Server machine. The Snap Server Manager basically finds the Snap box on your network and lets you assign an IP address to it. By default it looks for a DHCP server to get an IP address. Then by launching an Internet browser and typing in its IP address, you can configure the NAS. On the down side, the Snap Server doesn't make use of access control lists which means anyone can try and take control of this Snap Server if they happen to crack the password.

    Once we had the basics up and running it was time to configure iSCSI. On Snap Servers, an iSCSI Disk is based on an expandable, RAID-protected volume. To client machines it appears as a local SCSI drive. Unlike standard Snap Server volumes, Snap Server iSCSI disks can be formatted by the iSCSI client to accommodate different application requirements.

    Using the administration tools, you can create an iSCSI disk on an already existing volume. However the iSCSI disk is formatted, managed, and backed up from a client machine running iSCSI initiator software or hardware.

    Currently, the Snap Server supports the Windows initiators which runs on Windows 2000, 2003, and XP and is available for download from Microsoft at no cost. (Support for UNIX/Linux initiators will be available in a future release.)

    iSCSI disks are isolated from other resources on the Snap Server, because the file system of an iSCSI disk is different from the Snap Server's native file system, and also because they're managed by the client. We suggest you should either dedicate an entire Snap Server to iSCSI disks, or place the iSCSI disk on a separate volume, which is what we did.

    Performing a backup of iSCSI disk should be left to the clients. When attempting to back the iSCSI disk from a server, you may be backing up inconsistent data. This would happen if clients are connected to the iSCSI disk and were modifying the data. The client is the only one that can maintain the consistent state required for data integrity. It is also suggested not to use snapshots on volumes containing an iSCSI disk, as running a snapshot will disconnect clients that are writing to the iSCSI disk and the snapshot could contain inconsistent data.

    We downloaded the latest Microsoft iSCSI Software Initiator version 1.04a from Microsoft's Web site. After installing the Initiator software we set up the target portal which was basically the IP address of the Snap Server. The default socket was set to 3260. Once we added the target, we were able to view the two available disks we had previously created on the Snap Server. Then by logging in, we initiated an active session between our host machine and target NAS box.

    Under My Computer, the NAS box appeared just like any other hard disk. We were then able to transfer files to and from this disk. Microsoft's initiator software would also automatically restore the connection on reboot.

    Vendor: Lan1
    Phone: 1300 301 053
    Web: www.snapappliance.com
    Price: AU$8054 for 1TB model

  • Snap Appliance Server 4500
  • Adaptec ASA-7211C iSCSI Card
    Adaptec ASA-7211C iSCSI Card and Adaptec iSA 1500 Storage Array
    You may be as curious to find out what the performance difference is between the software-based Microsoft iSCSI initiator and a hardware-based iSCSI card. In theory an iSCSI card is supposed to avoid excessive CPU utilisation. We were going to test this out. Adaptec sent us an Ethernet over copper-based iSCSI card, which uses an onboard processor to process iSCSI functions for a complete TCP/ IP offload.

    We installed this card and drivers, which was easy enough to do. The drivers place an additional icon in the Windows Control Panel so you can go in and configure the card and add targets. We set the Snap Server as a target by giving it an IP address. (The Adaptec card can also scan for targets if you don't know the IP address of your target.) From here we expected to be able to login to the Snap Server. After thinking we had done something wrong during the install, we discovered that the Snap Server will only work with software initiators and TOE cards and not with true iSCSI cards. We really didn't have enough time to get a TOE card and run some tests, so unfortunately we couldn't do much with the Adaptec iSCSI card.

    Adaptec iSA 1500 Storage Array
    An engineer over at Adaptec informed us on that company's most recent arrival, the iSA-1500 external storage array. It acts as a true iSCSI target. The product has been available in the US for some time now and has only just hit our shores in the last few weeks. It comes in a 1RU form factor with four SATA drives (1TB) and is designed for small businesses and remote offices looking for a cost effective shared storage solution. It's a little more expensive than the Snap Appliance Server, but it's a true iSCSI target; together with iSCSI cards it should prove to be a fast and cost efficient storage system. Hopefully we get a look at one of these and compare it back to the Snap Appliance Server using TOE cards.

    Vendor: Adaptec
    Phone: 02 8875 7874
    Web: www.adaptec.com.au
    Price: interface card AU$999.90; disk array approx AU$15,000

  • Adaptec iSCSI Card and Storage Array
  • Connecting iSCSI

    We can make things a little bit more complex now. As we have mentioned earlier, iSCSI SANs can be connected over a wide area network with standard Ethernet equipment. However, when connecting to fibre channel SANs, an IP storage device is needed to convert the FC protocol to iSCSI.

    Both IP storage routers and switches allow users to extend the reach of the FC SAN and bridge FC SANs to iSCSI SANs. Having this functionality allows you to then perform FC-to-FC switching, FC-to-iSCSI switching, or FC-to-gigabit Ethernet switching.

    HP StorageWorks IP Storage Router 2122-2
    HP StorageWorks IP Storage Router 2122-2
    While visiting HP's Storage Centre in Rhodes, Sydney we had a look at HP's StorageWorks IP Storage Router 2122-2. This is HP's second generation IP storage router for small to medium-sized businesses and enterprise workgroups. This device helps migrate servers from a DAS to a SAN environment and consolidate storage.

    This IP Storage Router supports iSCSI bridging; it can bring storage resources to servers in the IP network. It also supports FC/IP for SAN extension, where it can extend the SAN infrastructure across WANs.

    The SR21222 iSCSI router has a list price of AU$17,045.

    Cisco MDS 9000 IP Services Module
    Cisco MDS 9000 IP Services Module
    Another impressive device is the MDS 9000 IP Services Module from Cisco. It supports the interconnection of remote SANs and extends SAN connectivity to IP enabled servers using FC/IP and iSCSI.

    It supports the full range of services available on other MDS 9000 family Switching Modules including virtual SANs, security, and traffic management. It includes eight hot swappable, small-form-factor-pluggable (SFP), LC gigabit Ethernet interfaces and all ports are configurable for both FCIP and iSCSI operation on a port-by-port basis.

    The MDS 9000 IP Services Module has a list price of AU$74,545.

    This article was first published in Technology & Business magazine.
    Click here for subscription information.

  • Connecting iSCSI
  • About RMIT Test Labs

    RMIT Test Labs
    RMIT IT Test Labs is an independent testing institution based in Melbourne, Victoria, performing IT product testing for clients such as IBM, Coles-Myer, and a wide variety of government bodies. In the Labs' testing for T&B, they are in direct contact with the clients supplying products and the magazine is responsible for the full cost of the testing. The findings are the Labs' own -- only the specifications of the products to be tested are provided by the magazine. For more information on RMIT, please contact the Lab Manager, Steven Turvey.

  • About RMIT Test Labs
  • Newsletters

    You have been successfully signed up. To sign up for more newsletters or to manage your account, visit the Newsletter Subscription Center.
    See All
    See All