Solid State Disk

Hard Disk Drives

Hot data is moved to SSD. . .

. . . And then returned to HDD when no longer hot.

SOLID STATE DISKS AND STORAGE TIERING

The solid-state disk market is growing as automated storage tiering takes hold, but it remains difficult for IT to determine which applications can benefit from SSD tiers.

TECHNICAL BULLETIN / COFFEE BREAK

A NEW DISK TIERStandard hard disk drives (HDDs) are spinning disks, and their mechanical actions can create a delay in data retrieval. Solid-state disks (SSDs) are an exciting improvement in disk technology - made of silicon memory chips and with no moving parts, SSDs have no rotational delay and near zero seek time, dramatically reducing response times. SSDs can be extremely valuable to applications that need fast performance.

SSDs offer a high-performance storage tier commonly referred to as Tier 0. If storage administrators know of data that will be in high demand, they can place it on SSD for fast performance, and place other data on HDD tiers. Since the effectiveness of SSD as a disk tier is based solely on how well the administrator can place data, it remains a solution that is useful for IT shops with a known performance problem on specific data sets. Most IT organizations are still unsure of whether SSDs can help them cost effectively.

THE NEXT EVOLUTION: AUTOMATIC TIERINGRecently, however, storage vendors have begun offering automated tiering along with better storage controller integration. Arrays can use SSDs dynamically, in concept like a dramatically larger cache. Instead of fully populating an enclosure with SSD drives, only a few SSDs are included; intelligent load-balancing algorithms identify “hot” data sets and move them to the SSD tier. When the data sets are no longer “hot,” they are moved back to a slower tier. Organizations can use fewer SSDs, and use them more efficiently. This type of automated deployment can spread SSD usage across more data and applications, eliminate the manual task of assigning data to an SSD tier, and reduce costs.

HOW DO I KNOW IF IT’S FOR ME?Because SSD storage is expensive, organizations want to be sure of its benefits before making an investment. Solutions that require administrators to manually track and move data offer fewer benefits than those that monitor and move data automatically. In addition, it is difficult to know which workloads need this class of performance, and whether the benefit is worth the cost. Will SSD implementation improve an application’s performance, or can that application achieve only a certain performance level no matter what disk type it uses? These answers are hard to ascertain and often the only way to find out is to try a different disk and see what happens.Knowing your workload characteristics will help you determine whether SSDs can help - unfortunately, these characteristics are not easy to distinguish.

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Best practices designed to alleviate hotspots may be at cross purposes with automated storage tiering with SSD, and may change in the future.

IDENTIFYING TIER-ABLE WORKLOADSAutomatic load balancing with SSDs can improve performance as long as the active data set fits in the SSD storage, but currently there are no tools in operating systems or storage arrays to identify active data set size. Storage arrays offer performance statistics, but that data describes only what happened for an entire LUN. Plus, there is no way to estimate how workload characteristics would change on a different storage medium. We can see where the storage system is busy, and we can see whether the application is waiting or not, but we cannot predict how much faster an application might perform with improved storage performance. These practicalities will be ironed out over time as the use of these technologies matures in operating systems, applications, and storage devices.

SOME ARE, SOME AREN’TSince data access hotspots make tiering beneficial, workloads with hotspots are more appropriate for SSD tiering than those without. For example, in virtual desktop infrastructures, a master image is created from which many desktops boot. When many desktops boot simultaneously, they work that image very hard — this is a naturally tier-able workload that is easy to identify because you know how it will be used. Many databases are also prone to hot spots.

SSD TIERS IMPACT APPLICATION DEVELOPMENT AND DEPLOYMENTThe ability of automated SSD tiers to handle data hotspots will impact application development and deployment. To optimize performance, developers and administrators follow best practices designed to minimize or avoid hotspot creation. These traditional best practices may not be appropriate when intelligent, automatic tiering is available. Because of automated tiering and SSDs, hotspots can be used to improve

performance and streamline storage costs. Without hotspots, administrators may be forced to place all of an application’s data on high-performance disks to ensure high performance. With hotspots, they may configure a small amount of high-performance SSD for some data, and less costly HDD and RAID types for the rest. The growth of SSD in the data center will alter many best deployment practices. EVALUATING TIERED STORAGE SOLUTIONSFor known tier-able workloads, SSDs are a great advancement. This popular and growing technology is more widely applicable with automated storage tiering. When considering these solutions, be sure you know what you are getting. What processes are automated - selection of data to move, actual data movement, or both? Is data movement only at the LUN level, or can more granular hot spots be accommodated? Does automated tiering impact or limit other advanced functions such as snapshots and replication, and do these data copies automatically move to the slower, less expensive tiers? SSDs offer significant benefits if they fit your workloads. However, if your workloads do not require high performance or are not tier-able, an investment in SSD’s may not be cost-efficient. While your system may perform well, it could be over-provisioned and under utilized for what the applications can leverage. As these technologies mature, more information should become available to help administrators know exactly which data sets will benefit from SSDs and automated tiering.

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