16 Dec 2004 10:49
With every server purchase comes a basic question: How much storage should you include with the new server purchase? For larger companies, you might have already addressed this question through the implementation of a storage area network. However, until fairly recently, centralised storage pricing has remained out of reach for many small- and medium- size organisations. Regardless of the specific situation at your company, one thing is certain: there are a lot of choices – and a lot of decisions to be made – in today's storage marketplace. In this article, I'll provide an overview of the current technology as well as its pros and cons. Use this article in conjunction with the Storage product comparison worksheet available for download from TechRepublic to get a better idea about the various options available for your business. In a future article, I'll provide a more in-depth view of each of these solutions. Storage options
There are three overall storage options that deserve consideration: direct-attached storage (DAS), network attached storage (NAS), and a storage area network (SAN). As you might expect, each option meets specific needs and has pros and cons that you must weigh before making a decision.
Each storage option is discussed in detail over the next two pages.
Direct-attached Storage | Network-attached Storage
Storage-Area Networks
Direct-attached storage
Just about anyone who's ever touched a server is familiar with DAS. DAS is the storage that fits right into the server or that hangs off the server. For example, storage connected to a server's external SCSI channel is also considered direct-attached storage.
DAS has enjoyed a long history and is still the storage of choice in many situations. With the fastest transfer rates between the disk system and the server, DAS is ideal in situations where fast disk access is demanded, although some new SAN equipment is starting to give DAS a run for its money in this department. Further, most applications have no trouble at all working in a DAS environment, so you don't usually need to worry about application issues and can focus on other areas that might cause problems instead.
Not all is rosy with DAS, however. First and foremost, IT managers must constantly contend with "space issues" which attend to answer to common questions:
Some options on the market help to alleviate the storage burden related to these questions, but either way, you still need to make best-guess estimates regarding storage and then expand that storage as unanticipated needs surface.
Second, you still have to manage most DAS on a server-by-server basis meaning that you need a mechanism in place to monitor server disk usage per physical unit. Not many IT managers relish the thought of running out of disk space in the middle of the day!
DAS is ideal in a number of situations:
Direct-attached Storage | Network-attached Storage
Storage-Area Networks
Network-attached storage
There are some times that you just need to throw storage on the network, accessible by a number of users, and call it a day. Enter network-attached storage (NAS). NAS installation is usually very simple. Like DAS, though, you need to answer some basic questions about how much storage you need for your particular task. Unlike DAS, NAS devices can usually be more easily expanded in the event you need additional capacity. For example, where a commonly available DAS device tops on in the 2-TB range, some NAS devices on the market can scale upwards of 200 TB. With a few exceptions, a NAS unit is a perfect device for situations where you just need to throw storage at a problem.
There are a couple of major exceptions to this rule of thumb: Most importantly, situations in which block-level access to data is required, such as for databases and Exchange information stores, are not appropriate for NAS units. Some NAS units do support these processes, but it's a case of forcing a tool into being appropriate for a job. Second, when you really need high-speed access to storage from a server, a NAS may not be appropriate since all data needs to traverse the network and is thus limited by the speed of your network.
There are two situations in which NAS devices really shine: first and foremost, Web serving, and in a very close second, general file storage. Both applications require significant disk space, but direct data access from a server is seldom required. Instead, since most data from these two types of stores is accessed over the network anyway, it doesn't matter that it's coming from a NAS vs. DAS hardware.
There's also another reason that NAS devices are really good for Web and file serving, but not for databases and Exchange stores. It has to do with what's called file-level vs. block-level data access. In file-level systems, data is accessed by a file name, as the name implies. In block-level systems, data is accessed using a block address, which is the location in which specific data is stored. In a client/server scenario, when you request a file from a file server, you're asking for a specific file and the server does the block read to get that data for you. Databases and Exchange stores have difficulty communicating in this way, so their stores are not appropriately stored on a NAS device. Databases and Exchange stores are more efficiently accessed using block-level functions available with DAS and SAN solutions.
Even though NAS is a great solution when you need to throw storage at a problem, it does have some drawbacks:
Direct-attached Storage | Network-attached Storage
Storage-Area Networks
Storage area networks
The grand pooh-bah of storage, the SAN is the most expensive storage option of the bunch, as well as the most complex. However, SANs provide capabilities not found in other solutions and, in the right situation, can actually end up saving a company some funds, even considering the expensive initial outlay.
SANs today come in two flavors: fibre channel, and iSCSI or IP-based SANs. Fibre channel is the most well known type of SAN, but over the last couple of years, iSCSI-based SANs have started to hit the market in a big way, mainly due to their good performance and much lower cost vs fibre channel.
SANs truly combine the best of both NAS and DAS storage. For example, with a proper implementation, you get a completely redundant storage network that is eminently expandable to hundreds of terabytes à la NAS, but you also get block-level access to the data just as you get with DAS. You also get access to data at a reasonable speed, making SANs good even for operations that require significant disk access. With a SAN, you also get centrally managed storage with the ability to provision space on-the-fly. Even better, with some implementations, you can even configure your servers with no internal storage and require that all systems boot directly from the SAN (fibre channel only).
With all of these great points, what are the downsides of a SAN? There are two major drawbacks to a SAN: cost and complexity, particularly when it comes to fibre channel implementations. A reasonable fibre channel SAN can start in the $50-60K range for just a terabyte or two of storage. On the other hand, newer SANs based on iSCSI might start in the $20-30K range, but aren't quite up to the performance levels of their fibre channel cousins. The difference in price is mostly due to iSCSI's ability to make use of off-the-shelf gigabit Ethernet hardware, whereas fibre channel requires specialised, expensive equipment.
Direct-attached Storage | Network-attached Storage
Storage-Area Networks
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