Sizing up storage alternatives

By Edwin Mier and Kenneth Percy
Network World, 03/19/01

Tech Insider It wasn't long ago that IT professionals shrugged off storage as a straightforward, albeit very boring, aspect of maintaining a computing infrastructure. But in the last few years, a push towards shared enterprise storage has given rise to several deployment options.

For instance, when does a network-attached storage (NAS) device do a better job storing hordes of enterprise data than a storage-area network (SAN)? And, how do these newer technologies compare with local storage, where a hard disk is directly accessed by a server via a SCSI cable connection?

How we did it
Scenarios
Feature: SAN meets WAN
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With the help of leading switch maker McData and leading server vendor Compaq - which both contributed components of our storage testing infrastructure - Network World Global Test Alliance partner MierCom kicked the competitive tires of these storage technology alternatives to see how performance varied across several common storage scenarios.

Our test bed was set up to loosely emulate file servers, Web servers, video servers and other application servers with regard to the data they routinely transfer to and from a storage location. We varied the storage location between a local SCSI-attached disk drive, a disk drive on a storage server across a Gigabit Ethernet LAN, and a disk drive in a SAN disk array connected over a Fibre Channel SAN.

Which setup worked best? It depends. Our tests show the right storage route to take depends on the storage network environment, the size of the files being stored or retrieved, the type of PCI bus connection, and how your users access the stored data.

Specifically, our tests indicate that:

The NAS environment - where data moves between a server "initiator" and a storage "target" over a Gigabit Ethernet network - can deliver better data-transfer performance than a SAN in certain cases, such as when file sizes are small.

SANs really outperform the NAS alternative when data reads or writes are sequential and file sizes are large, such as when a server is delivering streaming video, or when a server is backing up large data volumes.

When connecting a server to a SAN, performance is virtually the same whether the SAN adapter uses a 32-bit or 64-bit PCI-bus connection.

For a Gigabit Ethernet network interface card (NIC) in our NAS environment, performance was typically better via a 64-bit PCI-bus connection than a 32-bit PCI-bus connection. But the difference isn't much - only about 10% in our tests.

In all cases, writing data to a storage device takes more time and resources than reading it, and subsequently yields much lower data-transfer performance.

With random data reads - when there's no correlation between data from one read to the next - data-transfer performance is much lower than sequential reads of large data files in all scenarios we tested.

With random reads, data-transfer performance over a Gigabit Ethernet NAS is nearly as good as reading data from a local disk drive on a SCSI bus.

The data presented is, we believe, among the first such published storage-comparison results. Still, we caution readers to keep two points in mind.

First, these results are based on the particular equipment we deployed. A SAN disk array other than the Hitachi 5800 we used, for example, might exhibit different performance characteristics.

Second, due to the broad differences between SAN, NAS and SCSI environments, the results should not necessarily be viewed as perfect apples-to-apples comparisons. For example, while direct SCSI data storage exhibits the best data-transfer performance in some scenarios, it is not generally accessible by multiple servers concurrently, as stand-alone storage nodes in the NAS or SAN environments are.

Also, while we used an off-the-shelf Compaq server as a NAS storage target, we employed a specialized Hitachi Disk Storage Array as the target node in the SAN environment. There are specialized NAS storage nodes available, too, but our attempts to procure one for this testing were unsuccessful.

In the end

There are many other scenarios that could still be tested. For example, it would be interesting to see how data-transfer performance compares if disk storage was striped across multiple target disk drives, instead of just one. It would also be interesting to see how different, specialized storage nodes - such as those from Network Appliance in the case of NAS, or EMC in the case of SANs - perform by comparison. However, neither vendor was willing to participate in this novel test bed.

The data presented represents a first step toward quantifying which of the various storage alternatives does the best job for a particular set of requirements. As our testing shows, there are cases in which each delivers the best relative data-transfer performance.

It is clear that, as far as storage technologies go, one size does not fit all. Indeed, the moral of this story may be that users need to gain a better understanding of their storage needs before they sign on the bottom line for a SAN or NAS-based storage network.