Moon-Wook Oh

Durable write cache in flash memory SSD for relational and NoSQL databases

In order to meet the stringent requirements of low latency as well as high throughput, web service providers with large data centers have been replacing magnetic disk drives with flash memory solid-state drives (SSDs). They commonly use relational and NoSQL database engines to manage OLTP workloads in the warehouse-scale computing environments. These modern database engines rely heavily on redundant writes and frequent cache flushes to guarantee the atomicity and durability of transactional updates. This has become a serious bottleneck of performance in both relational and NoSQL database engines. This paper presents a new SSD prototype called DuraSSD equipped with tantalum capacitors. The tantalum capacitors make the device cache inside DuraSSD durable, and additional firmware features of DuraSSD take advantage of the durable cache to support the atomicity and durability of page writes. It is the first time that a flash memory SSD with durable cache has been used to achieve an order of magnitude improvement in transaction throughput without compromising the atomicity and durability. Considering that the simple capacitors increase the total cost of an SSD no more than one percent, DuraSSD clearly provides a cost-effective means for transactional support. DuraSSD is also expected to alleviate the problem of high tail latency by minimizing write stalls.

Lattice curvature induced substrate X-ray line broadening in InGaP/GaAs heterostructures

Abstract The interfacial coherency of InGaP/GaAs heterostructure wafers grown by the metal-organic chemical vapor deposition technique is examined and its effects on the X-ray line broadening of the GaAs substrate are investigated. Lattice mismatches are measured using both (400) symmetric and [511] asymmetric reflections. The strain-free chemical lattice misfit and the elastic strain are also calculated. The X-ray full width at half maximum (FWHM) of the substrate is measured, and its dependence on the degree of lattice mismatch is found. In order to understand this observation, measurements of lattice curvature are carried out using the automatic Bragg angle control method. It is found that the radius of curvature varies with the elastic strain and the substrate FWHM. Using the results obtained, the misfit stress is also determined. Various contributions to the substrate line broadening are separated out. Calculated line widths are shown to be in good agreement with measured values. It is thus concluded that the primary factor affecting the X-ray line width of the substrate is misfit stress induced lattice curvature. The results presented demonstrate that interfacial coherency is of major importance in influencing the structural properties of InGaP/GaAs heterostructures through elastic strain and lattice curvature.

Influences of substrate surface orientation on misfit accommodation in InGaP/GaAs heterostructures

Abstract The effect of crystallographic tilt of the substrate on the misfit strain and stress are investigated in this study. InGaP/GaAs heterostructure wafers are grown on two types of substrates with normal [100] orientation and 7 ° (±0.2 °) tilted from [100] toward [110] orientation. Lattice mismatches are measured using 511 asymmetric and 400 symmetric reflections. The lattice misfits and the elastic strains are also measured. The elastic strain-lattice misfit relation shows that larger strain is developed in the layers grown on the tilted substrate. The radius of curvature, R , plotted as a function of lattice misfit reveals that larger R , in general, is obtained from the sample with smaller misfit. It also shows that R is smaller in the tilted sample even when the lattice misfit is identical to that of the normal sample. It is also found that the misfit stress is larger in the tilted sample. The results presented show that the characteristics of misfit accommodation are improved in the layers grown on the tilted substrate.

Metal-clad ridge waveguide structure InGaP/InGaAIP visible laser diodes

InGaP/InGaAIP visible-light laser diodes with a metal-clad ridge waveguide structure have been fabricated. Compressively strained SCH-MQW structures were grown by low-pressure MOCVD and the lasing wavelength was 690 nm. The threshold current was 28 mA at 25°C for a 400 μm cavity and 5 μm stripe width. CW operation was obtained as the cleaved device by employing thick gold layer as a heat spreader. The light-output power versus CW current was linear up to a maximum light output power of 38 mW. Stable operation of the fundamental transverse optical mode was maintained up to 30 mW. These results show that this metal-clad ridge waveguide structure provides sufficient current confinement even under high light output power operation.