Unlike cache configurations, persistent, drive-based implementations let users tier SSDs along with traditional spinning disk. In an automated tiered storage environment, SSDs can be reserved for applications requiring the best performance, which means fewer SSDs are needed. Less-essential data that needs to be accessed quickly can be stored on relatively lower-cost, higher-capacity Fibre Channel or SATA drives, as opposed to being archived off-site or on tape. Automatically moving data back and forth between the solid state drives and disk tiers based on policies, such as frequency of access, can significantly reduce the cost of storing and managing the data.
The decision to implement the drives as cache or persistent storage maps goes directly back to the fundamental question: what do you want to achieve? Cache provides a significant performance improvement for the whole storage infrastructure, and does not require additional software or training. However, caching precludes easy tiering. When SSD is integrated as the top tier in a persistent storage environment, users can purchase only the number of solid state drives required to house the active blocks for their applications. They don't need to purchase SSDs for entire volumes.
3. SLC vs. MLC: It's what's inside the drive that counts
There are two basic flavors of SSD drives: those based on flash memory and those based on DRAM. Since the inception of SSDs, flash has changed the landscape and outpaced DRAM as the chosen datacenter technology. Although DRAM has performance benefits, flash is significantly faster than disk-based arrays, more affordable than DRAM to implement and is the widely offered format of most storage array vendors.
The flash found in SSDs is further broken into two categories: Single Level Cell (SLC) and Multi Level Cell (MLC). SLC flash is found predominantly in enterprise-class drives and, as the name implies, each data bit is stored in one cell. This format is associated with better reliability, improved longevity and better read/write cycles. MLC drives, while less expensive to manufacture, have slower transfer speeds, higher power consumption and lower cell endurance, and are typically found in consumer memory cards. The bottom line is more data is stored in each cell -- if a cell is lost, more data is lost along with it.
4. Software applications to maximize SSD efficiency
Increasing speed of operation and access to critical applications is the impetus for investing in solid state technology. So once the hardware decisions are made you have to address the software questions. Two storage virtualization technologies noted for their ability to make the SSD performance spike are thin provisioning and automated tiered storage. On top of these, you should also employ storage resource management (SRM) software to automatically track and report how much capacity is being used across tiers, and by the SSDs themselves. The SRM software should provide granular enough detail about utilization to take the guesswork out of SSD capacity planning.
This leads to a question everyone should ask: Are there hidden costs? Many solutions require investment in entire "bricks" and enclosures, which significantly increases the investment. Others allow you to purchase SSDs in smaller increments as the data set grows.
Additionally, will you be forced to predetermine volumes and applications for SSD technology? Can you use thin provisioning with the SSDs, or are you wasting capacity just to allocate storage? Thin provisioning means space is only consumed on these expensive drives when data is written, leaving as much space free as possible and the drives operating at peak performance. While thin provisioning is gaining popularity, few vendors offer the technology for SSDs.