Introduction to Networks Storage Area Networks

Network Storage A primary use of networking is to provide shared network storage In a standard server storage configuration, we typically have a server that contains some type of storage device that is shared so that clients on the network can access the shared storage on the server This configuration is relatively inexpensive to implement, but it has some serious drawbacks. For example, if the server goes down then the work stations on the network lose access to the shared storage device in that server One way to fix this is by implementing a storage area network, or SAN

SAN Network connections In a SAN, multiple servers are all configured to share a common storage device. Because they all share the same storage device, a failure of one of these servers doesn't necessarily mean that the data on the storage device becomes unavailable to the client systems If we implement clustering software, then these servers can take over for one another should one of them go down To accomplish this, we must have two networks defined. First, we have the standard production network to which all of our workstations are connected using a standard ethernet switch. The SAN servers will be connected to the same network, allowing them to communicate with the workstations

SAN servers Connected to Storage With a SAN, in addition to our regular production network, we also have a second network implemented This means that every SAN servers must have two network interfaces installed The second network, or Storage Area Network, connects each server to the shared storage, But, it does not have any workstations connected directly to it This means that the servers will sit on two networks. One will carry commands and replies from workstations to servers and the other will carry over commands and replies from servers to storage

Targets and Initiators There are two definitions we need to be aware of when working with a SAN - Targets and Initiators A SAN target is a storage device A SAN server is an initiator Generally speaking, servers will run initiator software to write to and read from targets All storage area networks will use a network connection to transfer data between the target and initiator

Fibre Channel and iSCSI There are two SAN technologies that are commonly used. The more expensive of the two is called Fibre Channel (FC), and the less expensive option is called iSCSI FC provides the best performance, while iSCSI costs the least to implement Regardless of which technology you use, they function in essentially the same way - The Production Network connects the server to the works stations. The SAN network connects each of the servers to the shared storage device

Fibre Channel Deployment To build a Fibre Channel SAN, you need to do the following: 1. Install a Fiber Channel adapter in each server that will access the shared storage on the SAN. 2. Deploy a Fibre Channel switch. 3. Connect each server to the Fibre Channel switch using the appropriate fiber optic cabling for the equipment you are using. 4. Deploy a shared storage device. Typically, this is an external RAID device containing multiple hard disk drives. The array enclosure has a Fibre Channel network adapter implemented that you can use to connect it to the Fibre Channel switch using the appropriate fiber optic cabling.

Fibre Channel Protocols Once physically configured, Fibre Channel uses SCSI protocols to manage the shared storage The storage devices on the Fibre Channel targets appear to the operating system on the initiators to be locally-attached SCSI hard disks, however, instead of controlling a locally-installed SCSI hard disk, the initiators send SCSI commands over the SAN fabric to manage the remote storage on the target One key difference between Fibre Channel and traditional SCSI is the fact that it supports multiple initiators. In a traditional SCSI implementation, there is only one SCSI controller (or initiator) on the SCSI bus. This controller manages all devices that are connected to the bus. However, in a Fibre Channel SAN, each server in the SAN fabric is an initiator and can send disk I/O SCSI commands to the shared storage device

iSCSI Deployment To create an iSCSI SAN you need to implement: 1. Dedicated Ethernet cabling (fiber optic or UTP) 2. A dedicated Ethernet switch 3. A dedicated Ethernet NIC in each server 4. A storage array that uses a standard Ethernet interface iSCSI is a network protocol that encapsulates SCSI commands within IP packets and transmits them over a standard Ethernet network. iSCSI is much less expensive to implement because it uses standard Ethernet hardware to create the SAN fabric

iSCSI Because iSCSI uses IP packets, the SAN could be implemented on your standard production network, with all of your other network hosts and devices However, this is a very bad idea. The performance of the SAN will be heavily impacted because the available network bandwidth would be shared between the SAN and the production network As a best practice, you should implement a dedicated network infrastructure (cabling, switches, and NICs) that is restricted only to iSCSI SAN traffic. (In much the same way that a Fibre Channel SAN is deployed)

iSCSI Protocols As with Fibre Channel, iSCSI storage devices are targets, while iSCSI servers are initiators The iSCSI initiator sends SCSI commands within IP packets to the iSCSI target over the network iSCSI uses port 3260 by default The storage devices on the remote iSCSI target appear to the operating system on the iSCSI initiator as locally-attached hard disks, effectively creating a SAN

FC vs iSCSI When choosing between iSCSI and Fibre Channel, you should be aware of several factors that may influence your decision: FC is more expensive and is usually harder to implement than iSCSi. FC requires specialized hardware and knowledge, while iSCSI can be implemented using standard Ethernet hardware and TCP/IP FC is currently faster than iSCSI. Using Gigabit Ethernet hardware, an iSCSI implementation can approximate the speed of the slowest FC SAN. FC has a distance limitation of 10km. With iSCSI, IP packets can be routed over many networks, accommodating longer distances. However, performance will be impacted as routing iSCSI packets will introduce latency Both iSCSI and Fibre Channel provide security features that that can be used to authenticate connections and encrypt data transmissions

Clustering Fault Tolerance Since the servers in a SAN share storage devices, they are commonly deployed in a clustered configuration With clustering enabled, multiple SAN servers can be grouped together in a cluster to provide a degree of fault tolerance Because all of the cluster data exists on the shared storage, there's no need to replicate data between servers. To users on the network, the cluster appears as a single file server. If one of the servers in the cluster goes down, the other devices immediately take over and continue to provide access to the files on the shared storage device This is called a failover, and it usually only takes a second or two to complete

Clustering Load Balancing Clustered SAN servers can also be configured to load balance If the storage provided by the cluster is heavily accessed by network users, it could potentially create a bottleneck and degrade performance Using a load-balancing cluster, the network traffic destined for the shared storage can be divided up and distributed between multiple devices in the cluster This can dramatically speed up performance of the storage system.