TrueTime clocks (TTCs) that offer accurate and reliable time within limited uncertainty bounds have been increasingly implemented in many clouds. Multi-region data stores that seek decentralized synchronization for high performance represent an ideal application of TTC. However, the co-designs between the two often failed to realize their full potential. This paper proposes K2, a multi-region data store that explores the opportunity of using TTC for distributed transactions. Compared to its pioneer, Google Spanner, K2 augments TTC's semantics in three core design pillars. First, K2 carries a new timestamp-generating scheme that is capable of providing a small time uncertainty bound at scale. Second, K2 revitalizes existing multi-version timestamp-ordered concurrency control to realize multi-version properties for read-write transactions. Third, K2 introduces a new TTC-based visibility control protocol that provides efficient reads at replicas. Our evaluation shows that, K2 achieves an order of magnitude higher transaction throughput relative to other geo-distributed transaction protocols while ensuring a lower visibility delay at asynchronous replicas.

NAND Flash-based devices (e.g., Solid State Disks) emerge as popular storage devices with ultra-fast performance in embedded systems. Previously, write amplification (i.e., page program operation amplification), resulting from restriction operations (i.e., out-of-place update and erase-before-write operations) in NAND Flash, is an essential metric to evaluate excessive page program operations in NAND Flash space and write endurance of NAND Flash. However, overhead caused by read and erase operations is not taken into account by write amplification. This typical overhead gives rise to bad impacts on data reliability in NAND Flash space as long as scaling of NAND flash memory process technology. In this paper, we propose a new evaluation metric called Bamp to explore all behaviors amplification (i.e., read, program, and erase) in NAND Flash-based devices. We obtain the value of Bamp in the term of energy consumption when user data is written to devices. Given a write-dominated workload condition, there are also excessive read and erase operations besides extra program operations in storage devices of embedded systems. Therefore, write amplification cannot comprehensively evaluate amplification behaviors in NAND Flash-based devices based on given amounts of written data in workload. We employ a measurement system to obtain the values of Bamp for NAND Flash-based devices in the term of energy consumption under a workload condition. From experimental results, Bamp can provide comprehensive analysis overhead per user data written in a NAND Flash-based device in comparison to write amplification. In addition, using Bamp, we can apply a right NAND Flash-based device in embedded systems.