Continuous data access: enterprise-level high availability using iSCSI.Storage Area Networks (SANs) can deliver data storage solutions that provide 99.999% data storage availability; but, originally, SANs were built on FC technology. However, for the majority of businesses, using FC to build a SAN is too expensive to deploy, too rigid in its components' geographical constraints CONSTRAINTS - A language for solving constraints using value inference. ["CONSTRAINTS: A Language for Expressing Almost-Hierarchical Descriptions", G.J. Sussman et al, Artif Intell 14(1):1-39 (Aug 1980)]. and too complex to configure See configuration. (software) configure - A program by Richard Stallman to discover properties of the current platform and to set up make to compile and install gcc. Cygnus configure was a similar system developed by K. according to according to prep. 1. As stated or indicated by; on the authority of: according to historians. 2. In keeping with: according to instructions. 3. real-time network needs. Industry leaders realized that a new network architecture was needed and developed a way to route SCSI SCSI in full Small Computer System Interface Once common standard for connecting peripheral devices (disks, modems, printers, etc.) to small and medium-sized computers. SCSI has given way to faster standards, such as Firewire and USB. over TCP/IP TCP/IP in full Transmission Control Protocol/Internet Protocol Standard Internet communications protocols that allow digital computers to communicate over long distances. . iSCSI paved pave tr.v. paved, pav·ing, paves 1. To cover with a pavement. 2. To cover uniformly, as if with pavement. 3. To be or compose the pavement of. the way for a new type of SAN, an IP-SAN, based on TCP/IP over Ethernet. With the right products, any business can now build a SAN that is simple, cost effective, highly scaleable and fast enough to exceed their performance requirements. [FIGURE 1 OMITTED] iSCSI Background Before examining how iSCSI can enable high availability Also called "RAS" (reliability, availability, serviceability) or "fault resilient," it refers to a multiprocessing system that can quickly recover from a failure. There may be a minute or two of downtime while one system switches over to another, but processing will continue. for an IP-SAN, let's review how an iSCSI storage target (volume) interfaces with the operating systems Operating systems can be categorized by technology, ownership, licensing, working state, usage, and by many other characteristics. In practice, many of these groupings may overlap. and file system on the host. This is especially important to understand since, with iSCSI, the host can conceivably con·ceive v. con·ceived, con·ceiv·ing, con·ceives v.tr. 1. To become pregnant with (offspring). 2. be in Florida while the data and the storage targets are in California. An IP-SAN and iSCSI work under the file system at the block level. This allows iSCSI to support virtually any application, including databases, backup and restore applications and Exchange. With traditional DAS or FC-SAN-attached disk drives or tape, a file system makes read and write requests via the server to the storage devices using a set of standard SCSI commands In SCSI computer storage, a command is the basic unit of communication. The SCSI command architecture was originally defined for parallel SCSI buses but has been carried forward with minimal change for use with Fibre Channel, iSCSI and Serial Attached SCSI. . The OS and file system have 100% control over these storage devices. iSCSI, like standard SCSI, is a block-based storage protocol layered underneath the file system. This means that an iSCSI volume appears as an additional disk drive when mounted by the OS. The iSCSI volume can be partitioned par·ti·tion n. 1. a. The act or process of dividing something into parts. b. The state of being so divided. 2. a. , named and formatted like a normal disk drive. The MS Windows screen in Figure 1 shows two new disk drives (Disk 2 and Disk 3) which are actually allocated to the server from an IP-SAN. An iSCSI initiator driver is available for all popular operating systems. Standards for the iSCSI initiator are governed by the IETF See Internet Engineering Task Force. IETF - Internet Engineering Task Force (Internet Engineering Task Force (c/o Corporation for National Research Initiatives (CNRI), Reston, VA, www.ietf.org) Founded in 1986, the IETF is a non-membership, open, voluntary standards organization dedicated to identifying problems and opportunities in IP data networks and proposing technical solutions to the ). The iSCSI initiator driver responds to file system SCSI commands that are targeted to the disk drives the initiator represents. The iSCSI initiator driver encapsulates these SCSI commands and data into iSCSI packets that are, in turn, encapsulated encapsulated Localized Oncology adjective Confined to a specific area, surrounded by a thin layer of fibrous tissue; encapsulation generally refers to a tumor confined to a specific area, surrounded by a capsule. See Islet encapsulation. into TCP/IP packets (see Figure 2). The TCP/IP packets are then routed very quickly over the Ethernet network, where they are delivered to an iSCSI storage target. The iSCSI storage target can be located on the same network within the building or halfway around the world. This iSCSI target has all the same attributes as a standard SCSI storage system. Once the iSCSI packet arrives at the iSCSI storage system representing the targets, the SCSI commands and data are decapsulated from the iSCSI/TCP/IP packet and are executed on the storage system. Once executed, the results are encapsulated back into iSCSI/TCP/IP and returned to the iSCSI initiator driver on the server where they are decapsulated and delivered to the SCSI layer and then the file system Automatic Multi-Path Failover Invoking a secondary system to take over when the primary system fails. Up-to-date copies of all required data and applications are maintained on the secondary system in order to respond immediately if the primary system becomes unusable. Also called "fallover." See replication. With the iSCSI Initiator The iSCSI initiator comprises layers that are key to providing multiple data paths between servers and iSCSI storage targets, like those provided by the iSCSI intelligent storage switch. The two main layers within the iSCSI initiator are the session and connection layers. [FIGURE 2 OMITTED] The first layer is the "session" layer, an upper layer responsible for maintaining the communication to the SCSI layer within the server. It also ensures proper order of SCSI commands and data to and from the server file system to the iSCSI storage target. SCSI commands are numbered in sequence as they are sent from the server. The iSCSI storage target arranges the SCSI commands according to their order, ensuring that commands are not lost, taken out of order or duplicated. Within every server there is usually only one iSCSI initiator but there can be more than one session established and running within a single initiator. For example, if there are two iSCSI storage targets being used by the server, then there would be one initiator with two sessions running. In Figure 3 there are two sessions running, which means that the server iSCSI initiator is using two unique iSCSI connected targets or disk drives/volumes. The second layer is the "connection" layer. The connection layer is the TCP/IP connection between the server and the iSCSI storage target which, in our case, is the iSCSI intelligent storage switch. The session layer can maintain several connections. In common applications there is only a primary iSCSI TCP/IP connection; but for 99.999 applications there is a primary iSCSI TCP/IP connection and an alternate connection (Figure 3). All iSCSI traffic between the server and storage system travels over the primary connection. In most IP-SAN deployments, this is a Gb Ethernet link and is more than fast enough to handle average server traffic. With some application servers, a 10/100 Ethernet link (100Mb) is adequate since most servers don't generate more than 50Mbs of sustained storage traffic. A Gb Ethernet link (1000Mb) is recommended to handle I/O (Input/Output) The transfer of data between the CPU and a peripheral device. Every transfer is an output from one device and an input to another. See PC input/output. I/O - Input/Output spikes if spikes result in I/O bursts greater than 50Mbs assuming a 50% overhead. [FIGURE 3 OMITTED] Because the iSCSI session is aware of alternate TCP/IP paths to the iSCSI storage target, it will automatically transfer traffic through an alternate TCP/IP connection. So, if one TCP/IP connection between a server and iSCSI storage target fails, the traffic is automatically routed through the alternate TCP/IP connection. Because the SCSI commands are numbered, the iSCSI storage target is able to arrange the commands received across multiple connections. [FIGURE 4 OMITTED] Demonstration of Server Multi-Path Failover To demonstrate how iSCSI failover functions, several companies participated in a third-party demonstration where five videos were streamed from five iSCSI storage targets on an iSCSI intelligent storage switch to five Microsoft Windows See Windows. (operating system) Microsoft Windows - Microsoft's proprietary window system and user interface software released in 1985 to run on top of MS-DOS. Widely criticised for being too slow (hence "Windoze", "Microsloth Windows") on the machines available then. 2003 hosts (Figure 4). Each host used the native Microsoft-supplied iSCSI initiator driver software. As reviewed earlier, the iSCSI session layer was responsible for maintaining the video stream to the video player application on the hosts. Each host had two TCP/IP Ethernet connections used by the iSCSI session. The primary connection was a 10/100 Ethernet CAT5 copper connection between the host and iSCSI intelligent storage switch via a switched LAN (Local Area Network) A communications network that serves users within a confined geographical area. The "clients" are the user's workstations typically running Windows, although Mac and Linux clients are also used. . The second connection was a wireless WI-FI connection between the host and iSCSI intelligent storage switch. The following statement reviews the failover test and results: "We were able to disconnect disconnect - SCSI reconnect the CAT5 cable from the hosts and the iSCSI session automatically routed the traffic over to the WI-FI connection. We were able to do video streaming See streaming video and video stream. to five hosts running iSCSI (wireless) going to an access point, then to a hub, then to the iSCSI intelligent storage switch (iSCSI to FCP-SCSI), then to a core-edge FC fabric, then to a virtual port, and finally mapped to an FC open-9 LUN on our enterprise class storage system. This forced failover test was extremely positive and fast enough to keep all five videos streaming." iSCSI Intelligent Storage Switch IP Take-Over for Enhanced High Availability IP take-over is key to enabling high availability and failover paths with the iSCSI intelligent storage switch. In the event an iSCSI intelligent storage switch is temporarily off line, the second storage switch attached to the same storage and the same host network can take over the IP addresses and data communication for the off-line switch. Both iSCSI intelligent storage switches are "active" servicing their assigned hosts but they can also provide a "passive" failover path for other hosts within the network. This is because both iSCSI intelligent storage switches maintain the configuration information of other storage switches within the cluster and monitor the heartbeat (1) A periodic signal generated by hardware for activation and/or synchronization purposes. See MHz. (2) A periodic signal generated by hardware or software to indicate that it is still running. 1. of their designated partner or partners. When a site or iSCSI intelligent storage switch goes off-line, iSCSI will terminate the host connections with the problematic site but maintain the iSCSI session within the host while waiting for the IP addresses for storage to be re-exposed. The other storage switch will now expose the IP addresses from the down site or iSCSI initiator will discover the re-exposed IP addresses and create a new connection, thus enabling the hosts to proceed with communication through a new iSCSI intelligent storage switch to the storage systems. [FIGURE 5 OMITTED] Conclusion IP-SAN can deliver 99.999% data storage availability. With the right products, any business can now build an IP SAN that is simple, cost effective, highly scaleable and fast enough to exceed the majority of their performance requirements. www.sanrad.com Zophar Sante is vice president, market development at SANRAD (Silicon Valley, CA) |
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