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Storage directly attached to a server is how disk capacity has historically been managed; networked storage, such as network-attached storage (NAS) systems and storage area networks (SAN) are a relatively recent phenomena. But thanks to boosts in server processing power and individual disk size, some users are finding that what's old is new again, especially when it comes to increasingly intelligent database applications.
Microsoft Exchange 2007 is the application leading the direct-attached storage (DAS) revival, which began earlier this year after Microsoft recommended that users, especially those with large mailboxes, use DAS, rather than networked storage to simplify management. A key feature, Cluster Continuous Replication (CCR), has been added to that version of the email application that allows Exchange to handle high-availability clustering so server nodes can have the same redundancy as SAN storage.
Matt Lavallee, director of technology for MLS Property Information Network, followed Microsoft's advice. He put Microsoft's SQL Server and Exchange on a Hewlett-Packard DL785 server directly attached to external Modular Smart Arrays (MSA). He said he did so mainly for performance, saying that "using the old JET database" -- which does random writes and sequential reads -- "it needs as much performance as possible."
DAS helps with performance tuning by isolating the application that MLS uses to serve 30,000 users, which allows for beefing up dedicated infrastructure for the application that needs it without breaking the bank for other applications sharing storage. The HP servers used by Lavallee have eight-way quad-core processors, and the external storage uses 3 Gbps SAS disks. "The more we know and control about the hardware, the more we can respond to performance issues," he says.
Having a direct connection between server and storage helps, too. According to Lavallee, "Even the best iSCSI and Fibre Channel get about 98% throughput -- this gets 100%. Where we're seeing the bottleneck now is the bus and the motherboard, not the server or the drive."
For MLS, putting the demanding Exchange application on its own island also cut costs. "The application, the server and the storage all cost us about $60,000," Lavallee says. "Using a separate SAN would have at least doubled that amount." MLS has also cut costs by using a 10 Gigabit Ethernet iSCSI SAN put together with Windows Storage Server, HP DL380 generation 5 servers with 10 GbE NICs and more external arrays.
DAS hardware has also gotten more attention from vendors of late, something Lavallee credits along with Moore's law for the DAS revival. For example, speedy SAS disks are generally confined to DAS use today because of cable length and other limitations not addressed until the recent ratification of the SAS-2 specifications. It was also over the past 12 months that HP added what it calls a dual domain option for MSA arrays that provide redundant paths throughout the infrastructure, from dual controllers and firmware on the drive provided by SAS vendors, to dual ports and controllers on the array box, and two lines between the array and the server.
Finding the balance between DAS and SAN
HP-developed DAS also made a big splash this year with another database application, the Oracle Exadata product announced at Oracle OpenWorld in September. The storage server is based on HP ProLiant hardware and contains 12 disk drives and a dual Intel quad-core processor.
Exadata Storage Servers can be packaged into the Oracle Database Machine, a preconfigured cluster with eight database servers, four Voltaire InfiniBand switches and 64 quad-core processors, Oracle Enterprise Linux and Oracle RAC software, and a 14-node storage server cluster with 168 disk drives. Oracle CEO Larry Ellison says the products will overcome performance bottlenecks for growing database repositories.
"Database applications can now do storage management," according to Forrester Research analyst Andrew Reichman, citing Exchange CCR and Oracle's Automatic Storage Management (ASM) as examples. "That's why there's more prevalence of DAS in the database world. Files need an intelligent storage system, but database applications can do the heavy lifting."
In fact, according to a new research report by Reichman, it's time to scrutinize the true value of SANs. "With low capacity utilization, the inability to prioritize application performance, long provisioning times and soaring costs, SANs haven't lived up to their promise," the report reads.
For now, vendors remain focused on both sides of the storage continuum. For certain applications, such as databases, including Exchange and centralized boot, HP offers DAS products, but "provide[s] the customer with a true evaluation of the choices they have available and the benefits of each," according to Lee Johns, director of marketing for entry storage and storage blades, HP StorageWorks.
And it's ditto for Dell. "Microsoft has made some significant enhancements to SQL and Exchange that make DAS an effective solution when customers are buying storage for a single application," said Matt Baker, storage strategist, Dell Storage. "DAS also makes sense for very high-throughput procedures like backup-to-disc, although SANs offer the benefits of consolidated backup and recovery and centralized management."
On the other hand, Sun Microsystems predicts an industry-wide consolidation between servers and storage, and has revamped its storage product lines to offer Sun servers running Solaris with ZFS and direct-attached disk.
One user of these Sun products, Jason Williams, chief technology officer at DigiTar, says he prefers DAS to keep staffing costs low. He said that DigiTar, an email monitoring and security service provider, "is a shop of generalists without the luxury" of specialized storage staff, and DAS simplifies deployments of new applications. According to Williams, "I load software on a server, use a script to create volumes and file systems, and I'm done."