Automated storage tiering: Higher performance AND lower cost?


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For Greg Folsom, CIO at Arnold Worldwide, simplicity is the key issue. According to Folsom, Dell Compellent systems are “drop-dead easy” to install and manage. Arnold Worldwide, a Boston-based ad agency, uses a three-tier strategy with two different storage policies. “These things are so easy that even I can be talked through managing them when our storage manager is away from the office,” he joked.

Chris Elam, Arnold Worldwide’s senior systems engineer, began using Dell Compellent’s default automated tiered storage policies but tweaked them over time. Dell Compellent’s Enterprise Manager utility helped Elam identify usage patterns. “Enterprise Manager helped us to see exactly how data is accessed in the system. With this information, we created a tier 1-2 policy for some apps and a tier 2-3 policy for other applications. We’ve been using the system for more than four years and we haven’t had to change the policies in a long time,” Elam said. New volumes are simply assigned to one of the policies at creation time.

Solid-state storage complements tiering

Xiotech Corp. offers another example of a “set-and-forget” AST implementation. Xiotech’s Hybrid ISE product combines SSD and hard disk drives in a sealed 14.4 TB 3U container. Of the 14.4 TB, 1 TB is SSD and the rest comprises 900 GB 10K rpm SAS drives (tier 2). Controller-level software, called Continuous Adaptive Data Placement, automatically manages data placement

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from the moment of deployment. Although the company provides a graphical ISE Analyzer utility to highlight I/O activity, in practice a user can’t adjust any of the parameters or configuration. The company says it designed Hybrid ISE to never need tuning.

Among the vendors offering more configurable architectures, NetApp Inc. stresses the ability to scale performance and capacity separately. The firm’s Flash Cache (PAM II) product is analogous to tier 0 SSD in other product lines. Though it can support multiple tiers, NetApp said in many cases the tiers can be simplified to two: Flash Cache and either tier 2 or 3. That’s because they’ve found data tends to be either “hot” or “cold” and rarely in between. Buffer cache is used to buffer write activity to avoid performance degradation.

Data block movement size is the most granular at just 4 KB. Although this architecture may require more flash disk than other systems (10% to 20% of total capacity), the elimination of relatively expensive tier 1 hard disks and spreading cold data across more SATA drives can result in the same performance at a lower total cost. Moreover, NetApp combines AST with deduplication and compression on the spinning disk for even greater space efficiency. Because data is managed through the WAFL file system and Data Ontap, it doesn’t need to be “rehydrated” when being elevated from a lower tier to tier 0 as the data becomes hot. The same automated storage tiering capabilities apply across all NetApp product lines.

CERN, the European Organization for Nuclear Research in Geneva, uses NetApp’s Flash Cache on Oracle RAC databases. “Prior to using Flash Cache, we had to size everything based on IOPS regardless of storage utilization,” said Eric Grancher of the CERN IT department. “Now, we can optimize both IOPS and capacity. We have moved from expensive Fibre Channel drives to less-expensive SATA drives. This has resulted in a substantial savings for the organization.” Grancher has found the NetApp system to be very adaptive to workloads resulting in simple management. His experience has determined that overall performance is better when the flash memory is in the storage rather than in the servers. “It makes more sense to have the stable NetApp systems cache the data rather than the database servers, which are restarted more frequently for patching or updates. A data cache on the storage server is already ‘warmed up’ and so eliminates the inevitable periods of poor performance we would suffer with cold server-based caches after each restart,” he said.

EMC Fully Automated Storage Tiering (FAST) is another example of a more configurable system. FAST has an install wizard that allows you to implement default configurations for simple deployment, which EMC says the majority of users find sufficient in most cases for “set and forget.” Other users tap into FAST Tier Advisor, a utility that collects usage statistics over time. Those statistics can be used to apply optimized policies for specific applications. Users can also set the size of the data movement block from 768 KB to 1 GB, depending on whether the reads tend to be random or sequential.

EMC recommends that users start with approximately 3% of capacity in tier 0, 20% in tier 1 and 77% in tier 3. Tier Advisor will track usage and, over time, tier 1 should be minimized as little more than a buffer between the higher and lower tiers. In any event, Tier Advisor lets users optimize any of the tiers based on actual usage patterns.

This was first published in May 2011

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