Performance of a two-headed disk system when serving database queries under the scan policy

Manolopoulos, Yannis; Kollias, John G.

doi:10.1145/68012.68017

Cited by 17 publications

(11 citation statements)

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“…Disks with one actuator and two R/W heads have been studied from the viewpoint of minimizing the seek distance by optimizing the R/W head separation [34,3,4,23]. For FCFS scheduling the optimum separation of 0.44657C cuts the expected seek distance for random disk accesses from C/3 [17] to 0.16C [3].…”

Section: Alternative Disk Drive Organizationsmentioning

confidence: 99%

Why specialized disks for composite operations may be unnecessary

Thomasian

2010

SIGARCH Comput. Archit. News

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Disk arrays with erasure coding such as RAID5 and RAID6 incur four and six disk accesses respectively for updating data and check blocks. The small write penalty can be reduced by the Read-Modify-Write (RMW) composite operations to update data and associated check blocks. The Disk Architecture with Composite Operation (DACO) is a proposal to eliminate the disk rotation associated with RMWs, by using a complex read/write head, which allows the writing of a block immediately after reading and modifying it without needing an extra disk rotation. We argue that the extra cost associated with DACO may not be justifiable, because it is not expected to have a significant impact on RAID performance. Furthermore an XOR capability is still required at the disk array controller for reconstructing missing data blocks. A duplexed Nonvolatile Storage (NVS) cache at the disk array controller provides the same reliability as magnetic disks and allows fast writes, i.e., writing to disk is considered completed as soon as data is written onto NVS. Deferring the destaging of data blocks from NVS allows these blocks to be be overwritten, obviating unnecessary disk writes. This also allows neighboring dirty blocks to be destaged in batches, so that a higher disk access efficiency is attained. Disks with multiple arms can also be used to make the processing of RMW requests more efficient, while disks with multiple R/W heads on one arm have little effect on RMW requests. In addition there are alternative methods to update check blocks, such as floating parities, parity logging, the reconstruct write method, log structured arrays, and variable scope parity protection.

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Section: Alternative Disk Drive Organizationsmentioning

confidence: 99%

Why specialized disks for composite operations may be unnecessary

Thomasian

2010

SIGARCH Comput. Archit. News

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“…These systems exist commercially and it has been proved that they perform better than conventional disks in terms of the distance traveled by the arm mechanism. For example, in [12] it has been shown that the gain in the distance traveled is near 50% if the heads are optimally spaced. This work's contribution is the derivation of the probability distribution for the number of stops of the moving arm.…”

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confidence: 99%

“…In the sequel, we focus on systems having two heads per surface separated by a fixed number of cylinders. For such systems, it has been proved that the optimum separation distance is 0.44657 x C if the FCFS scheduling policy is adopted [1], but if the SCAN scheduling algorithm is applied then this distance should be equal to [C/2] -1 or [C/2] -1 [12]. It is noted that the re-examination of analytical issues in two-headed disk systems is not trivial due to the additional physical constraints.…”

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Partial match retrieval in two-headed disks

Manolopoulos

Vakali

1995

Lecture Notes in Computer Science

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The performance of a disk with two heads per surface separated by a fixed number of cylinders is examined. We derive the probability distribution of arm stops, the expected number of stops as well as the expected number of cylinder clusters, i.e. the number of sets of consecutive compound cylinders. In comparison with a single-headed disk, it is shown that the performance gain may reach 50% on the average. 2 (b,a) (b,b) (b,c) (b,d) :2 Ca,b) (b,c) (c,d) (d,a) 3 (c,a) (c,b) (c,c) (c,d) 3 (a,c) (b,d)(c,a)(d,b) 4 (d,a) (d,b) (d,c)(d,d) 4 (a,d) (b,a) (c,b) (d,c)

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“…Στην αναφορά [MaKo89] υπολογίζεται και ο βέλτιστος αναμενόμενος χρόνος αναζήτησης για απόσταση γ -1 (["y -1] και | _^--lj) μεταξύ των κεφαλών. Ο χρόνος αναζήτησης για τον αλγόριθμο SCAN υπολογίζεται από τη σχέση :…”

Section: επισκόπηση βιβλιογραφίαςunclassified

Αναλυση Επιδοσης Συγχρονων Μαγνητικων Δισκων

Vakali¹,

Βακάλη²

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The evolution of computer systems technology is characterized by drastic changes and innovations. The main memory and processors evolved and progressed in a very fast speed, while the secondary memory (the third major factor in computer technology) proceeded in slower rates. Magnetic disks remained the main and dominant medium of permanent and non-volatile data storage, for the last three decades, despite various other technology proposals. Magnetic disks provide long lasting file storage and an additional level in memory hierarchy which is used as a virtual memory during the programs' execution. The present dissertation focuses in studying the performance of various modern magnetic disk drives. It examines various disk topologies and it includes specific surveys for the disks with two read/write heads per disk surface. The performance is measured by different metrics such as minimum, average and maximum seek times, number of disk cylinders accessed, number of requested clusters etc. χ ABSTRACT Comparisons axe made and the seek time is proven to be a very important factor needed in the overall performance measurement.Chapter 3 refers to various disk models for disk systems with two read/write heads per surface, where the heads remain always in fixed distance. There is a further examination of the Traveling Salesman Problem which is adapted to the two headed disk model. New formulae are derived for the servicing cost of various query types refering to a range of consecutive cylinder positions. In the case of partial match queries, both the expected number of the heads' stops and the expected number of the disk cylinder clusters are calculated for the two fixed headed disk model. Comparisons with the conventional one headed disk are made and the new results are discussed in detail.Chapter 4 considers the theoretical model of disks with two heads moving autonomously. Each head can sweep all cylinders of the disk surface and the requests are served by the nearerserver rule. The expected seek time is calculated analytically. There is also another servicing approach based on a greedy algorithm, in order to minimize the expected seek time. The greedy approach exploits the two headed scheme by placing one head on an appropriate position while the other head satisfies the request. Certain formulae for the expected seek time of both servicing approaches is derived.Chapter 5 presents in detail the modern systems of parallel disks, which comprise of either two disk drives (mirrored disks) or more than two drives (shadowed disks). In the present dissertation there is an analytic study of parallel disk systems where each disk has two heads either in fixed distance or in autonomous movement. A new mathematical approach for the calculation of seek distances traveled by the heads, is presented by using Markov chain tech niques. The study deals with the general case of shadowed disks by distinguishing seeks in dependent and independent ones. The dependency of seeks results in two different metrics for the expected seek distance. T...

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Performance of a two-headed disk system when serving database queries under the scan policy

Cited by 17 publications

References 24 publications

Why specialized disks for composite operations may be unnecessary

Why specialized disks for composite operations may be unnecessary

Partial match retrieval in two-headed disks

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