Staging Methods

When data are stored in the tertiary storage devices, the tape drives shall read them from the tapes using the input/output (I/O) operations. Due to the long delay in exchanging tapes, it is inconvenient to exchange a tape for each read/write access operation. Thus, the entire object or file is accessed from the tape drives well before they are being used (Federighi & Rowe, 1994; Kienzle, 1995; Pang, 1997). These accessed objects are temporarily stored in the magnetic hard disks as secondary storage level.

Constraint Allocation on Disks

Most existing storage servers store data stripes on magnetic hard disks. These magnetic hard disks are accessed by moving the disk heads to random disk tracks. A significant amount of overhead is spent in moving the disk heads across the disk tracks. The access time of a request would be significantly reduced if the seek time is reduced. In the normal placement of data stripes on disks being described in the two previous chapters, data stripes can be placed on any tracks with free space. There is not much consideration on the distance among data stripes of concurrent streams. Separation distances between data stripes of an object are not sufficiently constrained. Thus, the only guarantee on the upper bounds of access times is very high. Constraint allocation methods limit the available locations to store the data stripes. This helps to control the access time within media playback requirements. The data stripes are also evenly spread across the surface of the storage media. This reduces the overheads of serving concurrent streams from the same storage device. Therefore, the maximum overheads in accessing data from the storage devices, such as seek time, become lowered. In this chapter, we shall describe two constraint allocation methods that are designed for magnetic hard disks. These methods may also be applicable to other storage media that use the disk format. When many streams access the same hot object, the phase based constraint allocation supports more streams with less seek actions. We shall describe the phase based constraint allocation method in the next section. The region based allocation limits the longest seek distance among requests. After that, we describe the region based allocation method.

Deposition and Measurement of Nanoscale Complex Hydrocarbon Films on Magnetic Hard Disk Surfaces

Measurement of complex hydrocarbon film thickness on engineering surfaces such as magnetic hard disks is difficult if the film thickness is below the peak to valley distance of the surface roughness. Instrument sensitivity and detection accuracy for most of the common tools for this purpose is limited. We have developed a novel calibration technique that allows accurate measurement of such films within 5%–10% accuracy.

Electrochemical Tests of Corrosion of Magnetic Hard Disks With Overcoats of DLC

Two electrochemical techniques were used to investigate the corrosion of DLC coated magnetic hard disks: (1) potentiodynamic and potentiostatic anodic polarization and (2) electrochemical impedance spectroscopy. The objective was to determine the ability of DC polarization and AC polarization to detect the presence of porosity in the DLC. The results indicate that anodic polarization at high potentials (≈ +1.0V vs SCE) to cause pitting corrosion of the metal layers underneath the DLC might provide a rapid and quantitative estimate of the amount of porosity in the DLC. Measurement of the low frequency electrochemical impedance at intermediate potentials (≈ +0.5V vs SCE) appears to indicate the presence of porosity but further work is required to establish a quantitative relationship between the amount of porosity and the value of the low frequency impedance.