Advances in data storage devices and software intelligence have accelerated the use of tiering in the data center.
Storage tiering allows data to reside on media that provides an appropriate mix of cost and performance: an expensive, fast storage layer for recent or frequently accessed data; a slow, high-capacity and inexpensive storage layer for old or archival data; and a range in between.
SSDs and I/O acceleration devices enter the picture
Storage tiers often combine various spinning disks. A data center may use a Fibre Channel disk array for the top tier, and then small computer system interface attached storage drives, followed by Serial Advanced Technology Attachment (SATA) disk groups, and perhaps even tape drives as a long-term archival storage tier at the lowest rung. Disk drives provide ample reliability and capacity, but rotating magnetic media is limited by physical latency and seek time, even when grouped for simultaneous spindles and RAID configurations.
Solid-state memory is at least an order of magnitude faster than the best magnetic media. Flash technology includes solid-state drives (SSDs) and I/O accelerator cards, which are sometimes called solid-state accelerators (SSAs). Now that SSD and I/O acceleration devices are more affordable and reliable with better operating system support, organizations are deploying SSDs for the most demanding workloads.
SSD and I/O acceleration devices serve essentially the same purpose -- the difference is the data interface. SSDs, such as the Intel SSD DC S3500 Series, or SanDisk's Lightning Mixed-Use SAS devices, use flash storage configured with a standard SATA 6 Gbps interface. The host server or storage array treats the SSD like any other magnetic hard drive accessed using standard BIOS calls. By comparison, I/O accelerators like the Intel SSD 910 Series or Fusion-io's ioDrive devices use a PCI Express (PCIe) interface; the host system treats the device as an expansion card using a unique set of device drivers. Both device types can coexist on the same server.
Data storage devices on the PCIe interface offer somewhat faster performance. For example, Intel's DC S3700 800 GB SSD offers sequential reads/writes up to 500 MBps and 460 MBps respectively, and random reads/writes up to 75,000 IOPS and 36,000 IOPS. That is fast compared to magnetic drives, but it pales compared to accelerators like the 800 GB version of Intel's 910 card, which offers sequential reads/writes of 2 GBps and 1 GBps with random reads/writes of 180,000 IOPS and 75,000 IOPS.
Data migration is the key to successful storage tiering. There is little benefit to tiered storage if data cannot move between tiers as its value changes over time.
Data is typically migrated between storage tiers based on the information's age and activity. New data that is accessed frequently is typically stored on the fastest tiers. As data ages or its activity levels decline, the data can be migrated to a lower tier.
Administrators will find manually migrating data between tiers to be time-consuming and error-prone. It is critical to automate the migration process. Tiering and automated migration decisions traditionally occur in the storage array, but some operating systems now include storage tiering as a native feature. Microsoft Windows Server 2012 R2, for example, uses a "heat map" algorithm to migrate data according to its activity levels. A complementary storage pinning capability prevents selected files from moving. Administrators can "pin" critical but rarely used files or storage blocks to high-performance, top-tier data storage devices, rather than allowing the system to automatically drop the data to lower storage tiers.
Data protection strategies with tiered storage
As storage tiering in the enterprise becomes more prevalent, data protection practices warrant a review. Conventional backup, snapshot, replication or other data protection tools may not be able to follow specific files from tier to tier.
Storage tiering begets data protection tiering -- as data moves to a different tier, the applied data protection changes accordingly. For example, lower tiers may not be backed up or replicated as often as higher tiers. Important but less frequently accessed files may need additional protection, or may need to be pinned to a higher tier that uses more aggressive protection.
New storage devices like SSDs and I/O accelerators also open up data protection vulnerabilities. For example, non-volatile flash memory used in solid-state storage devices has a finite number of write cycles -- limited life expectancy -- which requires active wear leveling. Proactive device replacement and data rebuilds will help prevent unexpected SSD or SSA faults.
Stephen J. Bigelow asks:
How are storage devices tiered in your data center?
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