19-04-2012, 03:14 PM
JISC Technology and Standards Watch
san.doc (Size: 402 KB / Downloads: 39)
Introduction
Many organisations, including those in the HE/FE sector are finding that storage growth is increasing at an alarming rate and, when combined with a trend to require more servers to support storage, is leading to an unmanageable situation as far as storage management is concerned. The growth of distributed systems is also giving concern in many organisations as standards of support in a devolved environment are not always adequate. Consequently, consolidation of both servers and storage is looking very attractive.
Networked storage solutions (of which SANs and NAS are examples – see below) can offer increased flexibility for connecting storage, ensuring much greater utilisation of disk storage space and support for server consolidation (as storage and server capacity growth trends are no longer linked).
Installing a SAN is large and complicated undertaking, needing institutional management commitment and is more suited to environments where a large proportion of the institution’s data will reside on the SAN. NAS can provide “plug and go” solutions for file serving, but SANs are better able to support large corporate databases and provide enhanced resilience.
The Technology
Defining the Storage Technology: DAS, SAN, NAS
Traditionally, data storage resides on hard disks that are locally attached to individual servers. This is known as Direct Attached Storage (DAS). Although this storage may now be large (in the order of 100s of Gigabytes of data storage per server) the storage is generally only accessible from the server to which it is attached. As such, much of this disk space remains unused and plenty of ‘contingency’ has to be built into storage needs when determining server specification. In addition, if the server were to fail, access to the data held on those local disks is generally lost.
A Storage Area Network (SAN) is a separate “network” dedicated to storage devices and at minimum consists of one (or more) large banks of disks mounted in racks that provide for ‘shared’ storage space which is accessible by many servers/systems. Other devices, such as robotic tape libraries may be attached to the SAN. See Figure 1 for a representation of both DAS and SAN storage.
Disk Technologies
Enterprise class storage arrays used in SANs generally use Fibre Channel interfaces to internal FC-AL connections in the storage array with FC disks attached. More modestly priced storage arrays are available with an internal connection to either SCSI or ATA disks. Fibre Channel disks are designed for enterprise-class use and usually have top-end performance and reliability characteristics and thus attract premium prices.
However, in recognition of the fact that not all data needs to be treated equally (see the section Data Categorisation Strategy below), many SAN vendors now offer the option of storage arrays based on Serial ATA (SATA) disk technology [7]. Serial ATA disks are an evolution of the commodity parallel ATA (or IDE) disks used in PCs with a design intention of being at ATA-like price-points with SCSI-like performance. Such disks may not be suitable for mission critical enterprise database applications, but may have a role for less critical or low usage data. Indeed, some storage vendors now offer storage arrays that can accommodate various varieties of disk in the same cabinet e.g. FC and SATA. In such examples, the interface to the disk tray from the hosts/fabric is still Fibre Channel.
Strategic Fit and Industry Positioning
The take up of NAS and SAN solutions is rising and a NAS or SAN solution is cheaper to run than DAS. Total cost of ownership in the generic business sector has been found to be 55-60% cheaper than an equivalent amount of DAS storage. The industry as a whole reports an average support cost reduction of 80% (based on FTEs per MB storage) compared with supporting the equivalent DAS infrastructure. Further cost savings are seen following backup consolidation (typically 50-75% in tape drive consolidation) [13].
The benefits of using SAN and NAS technologies to consolidate storage are compelling [14]. The Butler Group believes that storage consolidation should be a primary objective for an organisation looking to optimise its IT infrastructure [14].
Fibre Channel SANs and IP-attached NAS are now established technologies. The usability of management tools is rapidly improving as they provide greater automation and become available for more platforms. In most cases, the savings and improvements in staff productivity, utilization rates and data availability more than justify the additional cost of installing SANs.
Storage Management
Storage management can encompasses several layers: management of the individual devices constituting the SAN (SAN Management), management of them as a virtual resource pool (Storage Virtualisation) and management/reporting of the data characteristics and growth patterns (Storage Resource Management).
SAN Management
SAN Management software is needed to actually configure and monitor the components of the SAN to enable them all to function together. It is directly concerned with enabling and controlling the movement of data within the SAN infrastructure.
SAN Management products are typically able to:
• Discover devices attached to the SAN – hosts, storage devices, switches and other fabric components
• Manage and monitor ports on the Fibre Channel switches
• Administer zoning on the switches to selectively enable access
• Administer LUN masking in the storage arrays to partition access to particular servers
• Monitor traffic levels and performance between components and through the switches
• Manage configuration changes within the SAN
Storage Virtualisation
Virtualisation is an overused term in computing and in the specific area of storage, there is also much scope for confusion over the use of the term “storage virtualisation”. Some storage arrays, for example, have in-built virtualisation features whereby the location of data and the disposition of storage LUNs are hidden.
Storage Virtualisation for the purposes of this report is an additional (optional) layer of storage management that can provide a centrally managed pool of storage with virtual volumes being made available to servers, as illustrated schematically in Figure 3 below. Such virtualisation solutions have the additional merit of operating in heterogeneous SAN environments, consolidating the storage devices from several vendors.