Storage Area Networks

Storage Components

Thus far, we have discussed devices being attached to the storage bus as though individual disks are attached. While in some very small, arbitrated loop configurations, this is possible, it is highly unlikely that this configuration will persist. More likely, storage devices such as disk and tape are attached to the storage fabric using a storage controller such as an EMC Symmetrix or a Compaq StorageWorks RAID controller. IBM would refer to these types of components as Fibre RAID controllers.

Highly Available Solutions

One of the benefits of storage area networks is that the storage can be managed as a centralized pool of resources that can be allocated and re-allocated as required. This powerful paradigm is changing the way data centers and enterprises are built, however, one of the biggest issues to overcome is that of guaranteed availability of data. With all of the data detached from the servers, the infrastructure must be architected to provide highly available access so that the loss of one or more components in the storage fabric does not lead to the servers being unable to access the application data. All areas must be considered including.

Introduction

SANs evolved to address the increasingly difficult job of managing storage at a time when the storage usage is growing explosively. With devices locally attached to a given server or in the server enclosure itself, performing day-to-day management tasks becomes extremely complex; backing up the data in the datacenter requires complex procedures as the data is distributed amongst the nodes and is accessible only through the server it is attached to.  As a given server outgrows its current storage pool, storage specific to that server has to be acquired and attached, even if there are other servers with plenty of storage space available. Other benefits can be gained such as multiple servers can share data (sequentially or in some cases in parallel), backing up devices can be done by transferring data directly from device to device without first transferring it to a backup server.

Fibre Channel Switched Fabric

In a switched fibre channel fabric, devices are connected in a many-to-many topology using fibre channel switches, as shown in Figure 4 below. When a host or device communicates with another host or device, the source and target setup a point-to-point connection (just like a virtual circuit) between them and communicate directly with each other. The fabric itself routes data from the source to the target. In a fibre channel switched fabric, the media is not shared. Any device can communicate with any other device (assuming it is not busy) and communication occurs at full bus speed (1Gbit/Sec or 2Gbit/sec today depending on technology) irrespective of other devices and hosts communicating.

Hubs

Hubs are the simplest form of fibre channel devices and are used to connect devices and hosts into arbitrated loop configurations. Hubs typically have 4, 8, 12 or 16 ports allowing up to 16 devices and hosts to be attached, however, the bandwidth on a hub is shared by all devices on the hub. In addition, hubs are typically half-duplex (newer full duplex hubs are becoming available). In other words, communication between devices or hosts on a hub can only occur in one direction at a time. Because of these performance constraints, hubs are typically used in small and/or low bandwidth configurations.

Bridges and Routers

In an ideal world, all devices and hosts would be SAN-aware and all would interoperate in a single, ubiquitous environment. Unfortunately, many hosts and storage components are already deployed using different interconnect technologies. To allow these types of devices to play in a storage fabric environment, a wide variety of bridge or router devices allow technologies to interoperate. For example, SCSI-to-fibre bridges or routers allow parallel SCSI (typically SCSI-2 and SCSI-3 devices) to be connected to a fibre network, as shown in Figure 8 below. In the future, bridges will allow iSCSI (iSCSI is a device interconnect using IP as the communications mechanism and layering the SCSI protocol on top of IP) devices to connect into a switch SAN fabric.

SAN Backup

Storage area networks provide many opportunities to offload work from the application hosts. Many of the devices in the SAN (either hosts or storage controllers) have CPUs and memory and are capable of executing complex code paths. In addition, any device can communicate with any other device, the SAN provides a peer-to-peer communication mechanism. This leads to such things as SAN-based backups. A storage controller can easily initiate the backup of a disk device to a tape device on the SAN without host intervention. In some cases, hybrid backup solutions are implemented where file system related information is provide by the host, but bulk copying of the data blocks is done directly from storage controller to tape device.

Conclusion

Storage area networks provide a broad range of advantages over locally connected devices. They allow computer units to be detached from storage units, thereby providing flexible deployment and re-purposing of servers and storage to suit current business needs. You do not have to be concerned about buying the right devices for a given server, or with re-cabling a datacenter to attach storage to a specific server.