Is Real-time Tiering the Next Big Thing in Storage?

Dot Hill is a storage array manufacturer that’s built a name for itself primarily in the OEM world. They’ve been manufacturing mid-range storage arrays for over 25 years, currently supporting HP, Lenovo, Stratus, Quantum and a number of other vendors, but also marketing storage systems under the Dot Hill brand. Their AssuredSAN line of storage arrays offer enterprise-class functionality for the mid-market data center, the newest of which is the Pro 5000. Storage Switzerland met with Dot Hill recently to discuss this system and what makes it unique, even when compared with traditional “tier 1” storage systems.

The Pro 5000 is built in 2U, rack-mounted, HA modules, with redundant, hot-swap components, automated failover and multi-path support. Base systems come in 2- or 3-tier configurations, with a combination of SSDs, 10K RPM SAS disk drives and 7K RPM ‘nearline’ SAS drives. Expansion units can be added for a total of 224TB usable capacity in 10U of rack space. Up to four ports can connect to each controller via iSCSI or 8Gb Fibre Channel and each system includes software for automated tiering, pooling, thin provisioning and system management. Options include snapshots, volume management and remote replication.

Real-time tiering

Storage tiering is a process that strives to optimize performance and capacity of the storage system by keeping the data most likely to be needed on faster storage and putting the rest on lower performing, lower cost storage. The concept is simple and a number of storage systems in the enterprise space have a tiering functionality, but as is the case with many storage concepts, the ‘devil is in the details’.

Typically, tiering algorithms are CPU-intensive so in order to reduce their impact on array performance, they’re run in a batch fashion and scheduled for times of lower I/O activity; often overnight. The problem with this strategy is that it’s optimizing today’s data for yesterday’s workloads. Dot Hill has developed a tiering process called RealStor that works in real-time, continually upgrading the data sets it stores in an incremental fashion, instead of waiting for the system to slow down and then run a larger batch tiering process.

Every 5 seconds it scans data and migrates blocks that meet the tiering criteria, based on a ranking of the blocks throughout the data set. But only a few blocks are moved at a time making the data migration between tiers a gradual, incremental process. In this way the Pro 5000 can run tiering in the background, continuously. And since it has a separate processor for the tiering calculations, this activity won’t impact performance.


The Pro 5000 uses a dedicated Intel processor for running non-data path functions, like the RealStor tiering algorithms and other overhead. The performance-related processing, like RAID calculations and actual I/O tasks are handled by FPGAs, so performance remains consistent as workloads change or the amount of data being moved between tiers increases.

This real-time tiering architecture makes the Pro 5000 an ideal storage system for dynamic, unpredictable workloads, such as those seen in virtual server environments. The result is companies can run more virtual machines on each hypervisor. In fact, Dot Hill reports having run IOMark tests supporting 240 VMs with a single Pro 5000 system.

Real-world uptime

With over 600,000 systems sold and 125,000 systems under maintenance contracts, Dot Hill is in a position to really understand what makes a storage system reliable. As other long-time storage manufacturers do, they collect data directly from their systems in the field, providing an ongoing stream of real-time data about how their systems are behaving in production environments. They continually roll this data into their reliability numbers, enabling them to know what their average system uptime is. They don’t just calculate their probability of failure, using a statistical model or a sample. They know what kind of uptime their systems are actually providing and consequently, what uptime a new customer can realistically expect.

Storage Swiss Take

Dot Hill isn’t exactly a ‘best kept secret’ in the storage industry, with 29 years of experience and a half a million storage arrays sold. But they’re probably not well known to many IT managers evaluating new disk arrays. Their new Pro 5000 system with real-time tiering may change that. Tiering is an established method for integrating flash into disk storage arrays and Dot Hill’s RealStor may be the next step the industry takes in tiering technology. When you consider this advancement is coming in a product designed for the mid-market, Dot Hill should soon be well known to many more IT professionals.

Dot Hill is a client of Storage Switzerland


Eric is an Analyst with Storage Switzerland and has over 25 years experience in high-technology industries. He’s held technical, management and marketing positions in the computer storage, instrumentation, digital imaging and test equipment fields. He has spent the past 15 years in the data storage field, with storage hardware manufacturers and as a national storage integrator, designing and implementing open systems storage solutions for companies in the Western United States.  Eric earned degrees in electrical/computer engineering from the University of Colorado and marketing from California State University, Humboldt.  He and his wife live in Colorado and have twins in college.

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