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Why SSDs are obsolete


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Solid State Drives - SSDs - are a product of convenience, not good architecture. Storage systems need to be re-architected to achieve the highest performance of NAND flash and, soon, byte-addressable non-volatile memory. Here's an example.




"SSDs were built because there are billions of SATA and SAS disk ports available. Filling some of those ports with SSDs promised to be quite profitable - a promise fulfilled in the last 5 years.


But now that non-volatile memory technology - flash today, plus RRAM tomorrow - has been widely accepted, it is time to build systems that use flash directly instead of through our antique storage stacks. The various efforts to decrease latency - SATA 3, NVMe, and others - still add layers of software between our applications and our data, creating complexity and wasting CPU cycles. A recent PhD thesis got me to thinking about this.


The Storage Bits take


SSDs are obsolete in an architectural, not functional, sense. They offer many advantages over disk drives, which will also continue to sell in the hundreds of millions for years to come, but the motivating idea behind SSDs - fill those SATA ports! - is less and less relevant to today's systems.


We need to incorporate flash and, soon, byte-addressable NVM memory, as they are, rather than making them seem like disks. This is no small effort, but with the slowing of processor performance increases, systems need to find performance elsewhere. The storage stack is ripe for disruptive improvement".


I posted the less technical parts of the article first above. The whole article is here: http://www.zdnet.com/article/why-ssds-are-obsolete/


The techies will love the disagreement in the discussion section at the bottom of the article, and no doubt repost the same arguments for and against the idea of succession of another architecture as proposed in the article. The author of the article credits a Doctoral thesis as being the spark of the article which can be found in a 1.33 MB pdf file here: http://research.cs.wisc.edu/adsl/Publications/yiying-thesis13.pdf


Again for the techies only, here is his abstract from the pdf:




Flash-based solid-state drives (SSDs) have revolutionized storage with their high performance. Modern flash-based SSDs virtualize their physical resources with indirection to provide the traditional block interface and hide their internal operations and structures. When using a file system on top of a flash-based SSD, the device indirection layer becomes redundant. Moreover, such indirection comes with a cost both in memory space and in performance. Given that flash-based devices are likely to continue to grow in their sizes and in their markets, we are faced with a terrific challenge: How can we remove the excess indirection and its cost in flash-based SSDs? We propose the technique of de-indirection to remove the indirection in flashbased SSDs. With de-indirection, the need for device address mappings is removed and physical addresses are stored directly in file system metadata. By doing so the need for large and costly indirect tables is removed, while the device still has its freedom to control block-allocation decisions, enabling it to execute critical tasks such as garbage collection and wear leveling. In this dissertation, we first discuss our efforts to build an accurate SSD emulator. The emulator works as a Linux pseudo block device and can be used to run real system workloads. The major challenge we found in building the SSD emulator is to accurately model SSDs with parallel planes. We leveraged several techniques to reduce the computational overhead of the emulator. Our evaluation results show that the emulator can accurately model important metrics for common types of SSDs, which is sufficient for the evaluation of various designs in this dissertation and in SSD-related research. Next, we present Nameless Writes, a new device interface that removes the need for indirection in flash-based SSDs. Nameless writes allow the device to choose the location of a write; only then is the client informed of the name (i.e., address)where the block now resides. We demonstrate the effectiveness of nameless writes by porting the Linux ext3 file system to use an emulated nameless-writing device and show that doing so both reduces space and time overheads, thus making for simpler, less costly, and higher-performance SSD-based storage. We then describe our efforts to implement nameless writes on real hardware. Most research on flash-based SSDs including our initial evaluation of nameless writes rely on simulation or emulation. However, nameless writes require fundamental changes in the internal workings of the device, its interface to the host operating system, and the host OS. Without implementation in real devices, it can be difficult to judge the true benefit of the nameless writes design. Using the OpenSSD Jasmine board, we develop a prototype of the Nameless Write SSD. While the flash-translation layer changes were straightforward, we discovered unexpected complexities in implementing extensions to the storage interface. Finally, we discuss a new solution to perform de-indirection, the File System De-Virtualizer (FSDV), which can dynamically remove the cost of indirection in flash-based SSDs. FSDV is a light-weight tool that de-virtualizes data by changing file system pointers to use device physical addresses. Our evaluation results show that FSDV can dynamically reduce indirection mapping table space with only small performance overhead. We also demonstrate that with our design of FSDV, the changes needed in file system, flash devices, and device interface are small."

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