10.12 RAID systems typically allow youto replace failed disks without stopping access to the system. Thus, the datain the failed disk must be rebuilt and written to the replacement disk whilethe system is in operation. Which of the RAID levels yields the least amount ofinterference between the rebuild and ongoing disk accesses? Explain youranswer.
Ans: In my judgement, RAID Level 1 suits best forthe given situation. It yields the least amount of interference between therebuild and ongoing disk accesses. In rebuilding period, RAID Level 1 copiesonly data from failed mirror disk. But in all other RAID levels, RAID copiesevery content available in the disk.
10.14 In the variable-length recordrepresentation, a null bitmap is used to indicate if an attribute has the nullvalue.a. For variable length fields, if thevalue is null, what would be stored in the offset and length fields?b. In some applications, tuples have avery large number of attributes, most of which are null. Can you modify therecord representation such that the only overhead for a null attribute is thesingle bit in the null bitmap?Ans:a. In variablelength record, null bitmap is used to represent the attributes of record whichhave null values.
In null bit map, the offset value is set to 1 and the length is1 byte.b. Yes, by storingthe null bitmap in the beginning of the record. It is useful for tuples having largenumber of attributes which are mostly null.
This record representation wouldsave storage space and extra work cost for retrieving attributes of the record.10.18 In the sequential fileorganization, why is an overflow block used even if there is, at the moment,only one overflow record?Ans: Inthe sequential file organization, the overflow block is used to insert recordsif no free space available to insert in sequential position. Then pointersadjust accordingly. Block is the smallest unit in the disk. Based on performanceperspective, storing record in overflow block is better than inserting multiplerecords in block.
The free space in the block may use for future insertions. 10.20If you have data that should not be lost on disk failure, and the data are writeintensive, how would you store the data?Ans: “Redundancy”is evolved to control disk failure. Redundancy means storing extra informationwhich normally not useful, but it can be used to rebuild the lost informationin disk failure. “Mirroring” is a redundancy technique which duplicates everydisk. If one disk fails, we can read thedata from another disk. So, by using mirroring we can avoid data loss on diskfailure.
10.21 In earlier generation disks thenumber of sectors per track was the same across all tracks. Current generationdisks have more sectors per track on outer tracks, and fewer sectors per trackon inner tracks (since they are shorter in length). What is the effect of sucha change on each of the three main indicators of disk speed?Ans:Thethree main indicators of disk speed are seek time, rotational latency and datatransfer rate. First, to access the data of a required sector, the arm needs tomove over the correct track first and then it needs to move to the given sector.The time taken for repositioning the arm is called seek time.
In modern disks withmore sectors per track have faster seek times than the disks which have lesssectors in older days. In earlier generation disks, the area of sector is more,so moving arm from one sector to another sector takes more time. Secondly, thetime used to access the sector under arm is called rotational latency. Becauseof less sector area in modern disks, rotational latency also decreases.Finally, the retrieving rate of data from or to stored disk is called datatransfer rate. Current disk systems are supporting maximum data transfer ratesper second.
Inner tracks data transfer rate is less than the outer tracksbecause of fewer sectors in the inner tracks. So, for this reason older generationdisks have more data transfer rate.