Abstract:
An apparatus is programmed with a plurality of programming instructions for generating, if possible, an alternative disk block allocation for a current disk block allocation that will yield improved overall access time for a sequence of disk accesses. In some embodiments, the programming instructions include the logic for tracing the sequence of disk accesses to determine the current disk block allocation, the logic for generating the alternative disk block allocation, if possible, using the trace results, and the logic for effectuating the alternate disk block allocation, if generated. In one particular embodiment, the logic for generating the alternative disk block allocation employs a random search approach, while in another embodiment, the logic for generating the alternative disk block allocation employs a heuristic approach.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of computer systems. More specifically, the present invention relates to the performance of disk drives of computer systems. 
     2. Background Information 
     In the past decade, performance of microprocessor based computer systems have increased dramatically. In particular, the operating speed of microprocessors have increased from the meager 16 MHz to well over 200 MHz. This trend is expected to continue without abatement. Correspondingly, while not as dramatic, performance of system and input/output (I/O) buses have also improved substantially, ensuring the microprocessors have adequate data to work with and kept busy. However, except for the improvement provided by buffering etc., the performance of disk drive has lagged behind. As a result, users are often deprived of the full benefit of the increased performance by the microprocessors. For example, when starting up an application, because the large majority of time is often spent on loading the application into memory from a disk drive, a user often does not see any difference in performance whether the user is using a system equipped with a 100 MHz microprocessor or a 200 MHz microprocessor. Thus, further improvement in disk drive performance is desirable, and as will be disclosed in more detail below, the present invention provides the desired improvement in disk drive performance as well as other desirable results, which will be readily apparent to those skilled in the art, upon reading the detailed description to follow. 
     SUMMARY OF THE INVENTION 
     An apparatus is programmed with a plurality of programming instructions for generating, if possible, an alternative disk block allocation for a current disk block allocation that will yield improved overall access time for a sequence of disk accesses. In some embodiments, the programming instructions include the logic for tracing the sequence of disk accesses to determine the current disk block allocation, and the logic for generating the alternative disk block allocation, if possible, using the trace results, and the logic for effectuating the alternate disk block allocation, if generated. In one particular embodiment, the logic for generating the alternative disk block allocation employs a random search approach, while in another embodiment, the logic for generating the alternative disk block allocation employs a heuristic approach. 
     In some embodiments, the apparatus is a computer system, with the programming instructions being packaged as a stand alone utility in one embodiment, and an integral part of an installation procedure of an application on another. In other embodiments, the apparatus is a disk drive with the programming instructions being integrated with the disk controller. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The present invention will be described by way of exemplary embodiments, but not limitations, illustrated in the accompanying drawings in which like references denote similar elements, and in which: 
     FIG. 1 is a simplified illustration of the present invention; 
     FIG. 2 illustrates one embodiment of the method steps of the present invention; 
     FIG. 3 illustrates one embodiment of the present invention; 
     FIG. 4 illustrates one embodiment of the operation flow of the tracer; 
     FIG. 5 illustrates one embodiment of the trace log; 
     FIG. 6 illustrates one embodiment of the operation flow of the reallocation optimizer; 
     FIG. 7 illustrates an alternative embodiment of the operation flow of the reallocation optimizer; 
     FIG. 8 illustrates in further details the concept of access run; 
     FIG. 9 illustrates in further details one embodiment of the &#34;long access run&#34; reallocation step; 
     FIG. 10 illustrates in further details one embodiment of the &#34;short access run&#34; reallocation step; 
     FIGS. 11-14 illustrate in further details one embodiment of the coalescing step; and 
     FIG. 15 illustrates one embodiment of a computer system suitable for programming with the embodiment of the present invention illustrated in FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, various aspects of the present invention will be described. However, it will be apparent to those skilled in the art that the present invention may be practiced with only some or all aspects of the present invention. For purposes of explanation, specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the present invention. However, it will also be apparent to one skilled in the art that the present invention may be practiced without the specific details. In other instances, well known features are omitted or simplified in order not to obscure the present invention. 
     Parts of the description will be presented in terms of operations performed by a computer system, using terms such as data, flags, bits, values, characters, strings, numbers and the like, consistent with the manner commonly employed by those skilled in the art to convey the substance of their work to others skilled in the art. As well understood by those skilled in the art, these quantities take the form of electrical, magnetic, or optical signals capable of being stored, transferred, combined, and otherwise manipulated through mechanical and electrical components of the computer system; and the term computer system include general purpose as well as special purpose data processing machines, systems, and the like, that are standalone, adjunct or embedded. 
     Various operations will be described as multiple discrete steps in turn in a manner that is most helpful in understanding the present invention, however, the order of description should not be construed as to imply that these operations are necessarily order dependent, in particular, the order of presentation. 
     Referring now to FIG. 1, wherein a simplified illustration of the present invention is shown. Illustrated on the top half of the figure are two simplified block representations 10a and 10b of a disk drive having eight blocks, block 0 through block 7. Denoted therein inside the blocks accessed by the three accesses of a simple sequence of disk accesses are the access identifiers, access 1, access 2 and access 3. The access pattern denoted in simplified block representation 10a illustrates the manner in which the three accesses are made, under an hypothetical disk block allocation, without optimization in accordance with the present invention, whereas the access pattern denoted in simplified block representation 10b illustrates the manner in which the same three accesses are made, under an alternate optimized disk block reallocation, wherein the data previously stored in block 7 have been moved to block 1, in accordance with the present invention. 
     Illustrated in the bottom half of the figure are illustrative estimates of the access times (in milli-seconds) for the three accesses under the unoptimized and optimized disk block allocations. As shown, and readily appreciated by those skilled in the art, the read times are substantially the same for all accesses under either allocation, however, under the optimized disk block allocation, significant amount of time savings will be achieved for seek and rotation times, as the block displacement between the successive accesses are much smaller, as compared to the unoptimized disk block allocation. In other words, by reallocating disk blocks, if it can be done, significant performance improvement can be achieved for a sequence of disk accesses. 
     As will be readily appreciated by those skilled in the art, the above simplified illustration is merely provided for ease of understanding. The problem addressed by the present invention is many times more complex than the simplified illustration. The lengths of the access sequences that are of interest are typically significantly longer. Additionally, many blocks are accessed multiple times in one sequence, and the multiple accesses are not necessarily in the same order. In other words, block x may be accessed n times in a sequence of interest, the first time after accessing block y, the second time after accessing block z, and so forth. Furthermore, not all blocks are available for re-allocation. Thus, the optimized disk block reallocation is seldom as simple as reallocating the disk blocks into a group of contiguous disk blocks, as illustrated by block representation 10b. 
     FIG. 2 illustrates one embodiment of the method steps of the present invention. As shown, for the illustrated embodiment, a trace is first generated for a sequence of disk accesses that are of interest, step 22. The trace includes information such as the disk locations accessed. Next, one or more attempts are made to generate an alternate disk block allocation that will yield improved overall access time, as determined by a cost function, step 24. An example of a simple cost function, for illustrative purpose, is T=d×c1+c2+c3, where T=access time, d is seek distance, c1 is seek time per unit of seek distance, c2 is rotation time, and c3 is read time. If at least one of the attempts is successful, the data are remapped into the alternate disk block allocation that yields the most improvement in overall access time, step 28. Otherwise, the original disk block allocation is retained, step 30. 
     FIG. 3 illustrates one embodiment of the present invention. As shown, for the illustrated embodiment, the present invention, disk block reallocator 36, includes tracer 38, reallocation optimizer 40 and reallocation engine 42. Tracer 38 is used to trace disk accesses made by application 32 that are of interest. For the illustrated embodiment, operating system 34 provides I/O read/write services for accessing disk drives, and application 32 utilizes these I/O read/write services when accessing disk drives. Furthermore, operating system 34 provides event notification services, and tracer 38 leverages on these services to trace disk accesses performed by application 32. Tracer 38 logs the trace results in access trace 44. 
     Reallocation optimizer 40 is used to generate, if possible, an alternate disk block allocation that will yield improved overall access time for a sequence of disk accesses, using trace results logged in access trace 44. If successful, reallocation optimizer 40 generates reallocation vector 46 setting forth the manner in which the disk blocks should be reallocated. Reallocation engine 42 is used to effectuate the reallocation as stipulated by reallocation vector 46. 
     While for ease of explanation, disk block reallocator 36 is illustrated as a &#34;separate block&#34; from application 32, as will be appreciated by those skilled in the art, disk block reallocator 36 may be implemented as an integral part of application 32 or separate from application 32. For example, in an integral implementation, disk block reallocator 36 may be implemented as part of the installation procedure to be executed after post-installation of an application to ensure the data required during start up will be accessible through a sequence of deterministic disk accesses to a group of optimally allocated disk blocks. On the other hand, in a separate implementation, disk block reallocator 36 may be implemented as a stand alone utility to be invoked by a user and/or an application. Furthermore, as will be readily appreciated from the description to follow, to those skilled in the art, for the stand alone implementation, disk allocator 36 along with the required operating system support services may be implemented as an integral part of a disk controller. 
     FIG. 4 illustrates one embodiment of tracer 38. As shown, for the illustrated embodiment, upon invocation, tracer 38 registers itself with operating system 34, denoting its interest in disk accesses, in particular, the locations accessed, step 52. Upon registering itself, tracer 38 waits for the requested information to be returned from operating system 34, and logs the access data as they are returned, as long as the trace period has not elapsed, steps 54-58. Tracer 38 may be encoded or dynamically provided with the length of the trace period. 
     FIG. 5 illustrates one embodiment of access trace 44. As shown, for the illustrated embodiment, access trace 44 includes a number of access records 62. Each access record 62 includes an access identifier 64 identifying the access sequence number, and the disk blocks accessed 66. 
     FIG. 6 illustrates one embodiment of reallocation optimizer 40. As shown, for the illustrated embodiment, upon invocation, reallocation optimizer 40 processes the trace data recorded in access trace 44 to obtain the current disk block allocation for the sequence of disk accesses of interest, i.e. the disk locations accessed, and in turn generates the cumulative access time for the current disk block allocation, step 72. Next, reallocation optimizer 40 notes that current disk block allocation as the optimal disk block allocation, and the current cumulative access time as the optimal cumulative access time, step 74. 
     Having done so, reallocation optimizer 40 generates an alternate disk block allocation with randomized incremental changes, step 76. Randomized incremental changes may be synthesized in accordance with any number of such techniques known in the art. Using the generated alternate disk block allocation, reallocation optimizer 40 determine a new cumulative access time for the sequence of disk accesses of interest, step 78. If the generated alternate disk block allocation yields improved overall access time, i.e. the new cumulative access time is better than the &#34;optimal&#34; cumulative access time, reallocation optimizer 40 notes the generated alternate disk block allocation as the optimal disk block allocation, and the new cumulative access time as the optimal cumulative access time, step 82. Otherwise, step 82 is skipped. 
     Steps 76-80 and conditionally step 82 are repeated until a predetermined condition for terminating the search for alternate disk block allocation that yields improved overall access time has been met. A variety of termination conditions may be employed. For example, reallocation optimizer 40 may be encoded or dynamically provided with a parameter delimiting the &#34;length&#34; of search, in terms of total evaluation or elapsed time, number of alternate reallocation schemes evaluated, etc. At the conclusion of the search, reallocation optimizer 40 generates reallocation vector 46 denoting the reallocation to be performed, based on the optimal disk block allocation, step 86. For the illustrated embodiment, if the optimal disk block allocation was never updated, i.e. no disk block allocation yielding improved overall access time was found, reallocation vector 46 is a null vector. 
     FIG. 7 illustrates an alternate embodiment of reallocation optimizer 40. As shown, for the illustrated embodiment, upon invocation, similar to the earlier described embodiment, reallocation optimizer 40 processes the trace data recorded in access trace 44 to obtain the current disk block allocation for the sequence of disk accesses of interest, i.e. the disk locations accessed, and in turn computes the cumulative access time for the current disk block allocation, step 91. Next, unlike the earlier described embodiment, reallocation optimizer 40 coalesces the disk blocks, step 92. Coalescing the disk blocks may be performed in any one of a number of known techniques. One approach will be briefly described later. 
     Having coalesced the disk blocks, reallocation optimizer 40 proceeds to analyze the trace data and groups the disk accesses into access runs, step 94. FIG. 8 illustrates the concept of access runs. Illustrated therein is a hypothetical sequence of disk accesses, access 1 through access 9, against the enumerated blocks in the order shown. For this hypothetical sequence of disk accesses, blocks 2, 7 and 5 are always accessed as a &#34;run&#34;. These blocks are accessed in order during access 1 through access 3, and then during access 5 through access 7. Likewise, blocks 8 and 10 are also considered as a &#34;run&#34;, except it is accessed only once. Block 4 is a &#34;run&#34; with a run length of one. 
     Return now to FIG. 7, having grouped the accesses into access runs, reallocation optimizer 40 reallocates the disk block on an access run basis. For the illustrated embodiment, the &#34;longer&#34; access runs are reallocated first, step 96, before the &#34;shorter&#34; access runs are reallocated, step 98. &#34;Longer&#34; access runs are access runs with run lengths greater than a predetermined run length threshold (L1), whereas &#34;shorter&#34; access runs are access runs with run length shorter than L1. The value of L1 is application dependent, and is empirically determined. In one embodiment, L1 is set to 64. After, both the &#34;longer&#34; as well as the &#34;shorter&#34; access runs have been reallocated, reallocation optimizer 40 generates reallocation vector 46 as the earlier described embodiment. 
     FIGS. 9-10 illustrate one embodiment each for reallocating the &#34;longer&#34; and &#34;shorter&#34; access runs. As shown in FIG. 9, for the illustrated embodiment, reallocation optimizer 40 reallocates the &#34;longer&#34; access runs to contiguous disk regions at both ends of a disk, alternating between the two ends, until all &#34;longer&#34; access runs have been reallocated. At step 152, reallocation optimizer 40 determines if there are still &#34;longer&#34; access runs to be reallocated. If the determination is affirmative, for the illustrated embodiment, reallocation optimizer 40 reallocates the longest of the remaining &#34;longer&#34; run to the &#34;top most&#34; portion of the disk region, step 154. At step 156, reallocation optimizer 40 again determines if there are still &#34;longer&#34; access runs to be reallocated. If the determination is affirmative, for the illustrated embodiment, reallocation optimizer 40 reallocates the longest of the remaining &#34;longer&#34; run to the &#34;bottom most&#34; portion of the disk region, step 158. Steps 152-158 are repeated until all &#34;longer&#34; access runs have been reallocated. As steps 154 and 158 are repeated, the &#34;top most&#34; portion bound reallocations are reallocated in a &#34;top down&#34; manner, whereas the &#34;bottom most&#34; portion bound reallocation are reallocated in a &#34;bottom up&#34; manner. In other words, the &#34;center&#34; portion of the disk region is left unallocated at the end of the &#34;longer&#34; access run reallocation. 
     As shown in FIG. 10, for the illustrated embodiment, reallocation optimizer 40 reallocates the &#34;shorter&#34; access runs, by first arbitrarily picking one of the &#34;shorter&#34; access runs, step 162. Then the successor &#34;shorter&#34; access runs to the selected &#34;shorter&#34; access run are reallocated near the selected &#34;shorter&#34; access run based on their likelihood of occurrence, i.e. the frequency of occurrence of the successor &#34;shorter&#34; access run, steps 164-166. A successor access run is simply an access run that follows the selected access run. Steps 164-166 are then repeated until all successor access runs to the selected access run are reallocated. Then, the entire &#34;shorter&#34; access run reallocation process, i.e. steps 162-166 are repeated until all &#34;shorter&#34; access runs have been reallocated. 
     The two approaches to reallocating &#34;longer&#34; and &#34;shorter&#34; access runs are complementary to each other. Together the two approaches provide the advantage of reducing the access time of the &#34;shorter&#34; access runs, since they are all concentrated at the &#34;center&#34; portion of the disk region, and the advantage of spreading the higher cost of moving to the end portions of the disk region over a larger number of accesses, since a number of successive accesses will be made at the end portions once the actuator is moved there. 
     Returning now to the topic of coalescing disk blocks of a disk drive. FIGS. 11-14 illustrate one approach for achieving the desired coalescing of disk blocks. As shown, for the illustrated approach, reallocation optimizer 40 first determines if both the smallest unused disk region 1  as well as the smallest used region are smaller than a predetermined size, step 102. The value of the predetermined size is also application dependent, and empirically determined. In one embodiment, a value of 64 is also used for the predetermined size. If the determination is affirmative, reallocation optimizer 40 reallocates all or a portion of the smallest used disk region into the smallest unused disk region, step 104 (see also FIG. 12). Steps 102-104 are repeated until either the smallest unused disk region or the smallest used disk region is greater than or equal to the predetermined size. Together, these two steps have the effect of filling up the small &#34;in-between&#34; unused disk regions, and eliminate the small &#34;in-between&#34; used disk regions, as illustrated by FIG. 12. 
    
     Next, for the illustrated approach, reallocation optimizer 40 determines if the smallest unused disk region is smaller than the predetermined size, step 106. If the determination is affirmative, reallocation optimizer 40 reallocates one or both of the two used disk regions bounding the smallest unused disk region, by shifting one towards the other, or both towards each other, step 108 (see also FIG. 13). Steps 106-108 are repeated until the smallest unused disk region is greater than or equal to the predetermined size. Together, these two steps have the effect of eliminating the &#34;in-between&#34; small unused disk regions as illustrated by FIG. 13. 
     Next, for the illustrated approach, reallocation optimizer 40 determines if the smallest used disk region is smaller than the predetermined size, step 110. If the determination is affirmative, reallocation optimizer 40 reallocates the smallest used disk region, by shifting it towards the closest neighboring used disk region, step 112 (see also FIG. 14). Steps 110-112 are repeated until the smallest used disk region is greater than or equal to the predetermined size. Together, these two steps have the effect of eliminating any &#34;in-between&#34; used disk regions as illustrated by FIG. 14. 
     While at first brush, the two embodiments for generating an alternate disk block allocation that yields improved overall access time appear to be very different, as will be appreciated by those skilled in the art, they are really two species of a genus of approaches to practically and optimally solving the cost function of overall access time, expressed in terms of disk block allocation, i.e. Min. C{b1, b2, . . . , bn}, where C{ } is the cost function of overall access time, and (b1, b2, . . . bn) is a set of disk block allocation. 
     FIG. 15 illustrates one embodiment of a computer system suitable for programming in accordance with the teachings of the present invention described above. As shown, for the illustrated embodiment, computer system 200 includes processor 202, processor bus 206, high performance I/O bus 210 and standard I/O bus 220. Processor bus 206 and high performance I/O bus 210 are bridged by host bridge 208, whereas I/O buses 210 and 212 are bridged by I/O bus bridge 212. Coupled to processor bus is cache 204. Coupled to high performance I/O bus 210 are system memory 214 and video memory 216, against which a video display is coupled. Coupled to standard I/O bus 220 are disk drive 222, keyboard and pointing device 224 and communication interface 226. 
     These elements perform their conventional functions known in the art. In particular, disk drive 222 and system memory 214 are used to store a permanent and a working copy of the programming instructions for effectuating the teachings of the present invention, when executed by processor 202. Disk drive 222 and system memory 214 may also be used to store similar copies of application 32 and operating system 34. The constitutions of these elements are known. Any one of a number of implementations of these elements known in the art may be used to form computer system 200. 
     As described earlier, for the standalone implementation, the present invention may also be practiced as an integral function of a disk controller. 
     Thus, a method and an apparatus for improving disk drive performance have been described. While the method and apparatus of the present invention have been described in terms of the above illustrated embodiments, those skilled in the art will recognize that the invention is not limited to the embodiments described. The present invention can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of restrictive on the present invention.