Operating System Support for Database Management

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virtual memory e g Pilot 16 may exactly which block it will access. read X provide a solution to this problem next Unfortunately this block is not. This topic is examined in detail in necessarily the next one in logical file. 1, main memory , Section 6 order Hence there is no way for an. OS to implement the correct prefetch, strategy , cache 2 2 LRU R e p l a c e m e n t. Although the folklore indicates, D that L R U is a generally good tactic. for buffer management it appears to, 2 4 Crash Recovery. An important DBMS service is to, perform only marginally in a data provide recovery from hard and soft.
base environment Database access crashes The desired effect is for a. 9 in I N G R E S is a combination of unit of work a transaction which. 1 sequential access to blocks may be quite large and span multiple. which will not be rereferenced files to be either completely done or. 2 sequential access to blocks look like it had never started . which will be cyclically rerefer The way many DBMSs provide. enced this service is to maintain an inten , 3 random access to blocks which tions list When the intentions list is. will not be referenced again complete a commit flag is set The. I 4 random access to blocks for last step of a transaction is to process. disk I 3 the intentions list making the actual, which there is a nonzero prob . ability of rereference updates The DBMS makes the last. Fig 1 Structure o f a Cache operation idempotent i e it gener . Although L R U works well for case ates the same final outcome no mat . 4 it is a bad strategy for other situ ter how many times the intentions. ations Since a DBMS knows which list is processed by careful program . blocks are in each category it can ming The general procedure is de . 2 1 Performance use a composite strategy For case 4 scribed in 6 13 An alternate pro . The overhead to fetch a block it should use L R U while for 1 and 3 cess is to do updates as they are. from the buffer pool manager usu it should use toss immediately For found and maintain a log of before. ally includes that of a system call and blocks in class 3 the reference pattern images so that backout is possible . a core to core move For U N I X on is 1 2 3 n 1 2 3 Clearly During recovery from a crash the. a PDP 11 70 the cost to fetch 512 L R U is the worst possible replace commit flag is examined If it is set . bytes exceeds 5 000 instructions To ment algorithm for this situation the DBMS recovery utility processes. fetch 1 byte from the buffer pool Unless all n pages can be kept in the the intentions list to correctly install. requires about 1 800 instructions It cache the strategy should be to toss the changes made by updates in. appears that these numbers are immediately Initial studies 9 sug progress at the time of the crash If. somewhat higher for U N I X than gest that the miss ratio can be cut the flag is not set the utility removes. other contemporary operating sys 10 15 by a DBMS specific algo the intentions list thereby backing. tems Moreover they can be cut rithm out the transaction The impact of. somewhat for VAX 11 780 hardware In order for an OS to provide crash recovery on the buffer pool. 10 It is hoped that this trend to buffer management some means manager is the following . ward lower overhead access will con must be found to allow it to accept The page on which the commit. tinue advice from an application pro flag exists must be forced to disk. However many DBMSs includ gram e g a DBMS concerning the after all pages in the intentions list . ing I N G R E S 20 and System R 4 replacement strategy Designing a Moreover the transaction is not re . choose to put a DBMS managed clean buffer management interface liably committed until the commit. buffer pool in user space to reduce with this feature would be an inter flag is forced out to the disk and no. overhead Hence each of these sys esting problem response can be given to the person. tems has gone to the trouble of con submitting the transaction until this. structing its own buffer pool man 2 3 Prefetch time . ager to enhance performance Although U N I X correctly pre The service required from an OS. In order for an operating system fetches pages when sequential access buffer manager is a selected force out. OS provided buffer pool manager is detected there are important in which would push the intentions list. to be attractive the access overhead stances in which it fails and the commit flag to disk in the. must be cut to a few hundred instruc Except in rare cases I N G R E S at proper order Such a service is not. tions The trend toward providing or very shortly after the beginning present in any buffer manager. the file system as a part of shared o f its examination of a block knows known to us . 413 Communications July 1981, of Volume 24, the ACM Number 7. a character array object The follow tures as efficiently as those they cre . COMPUTING ing subsections explain why ate themselves 2 It is our feeling. PRACTICES 3 1 Physical Contiguity, that OS designers should contem . plate providing DBMS facilities as, The character array object can lower level objects and character ar .
usually be expanded one block at a rays as higher level ones This phi . 2 5 Summary time Often the result is blocks o f a losophy has already been presented. Although it is possible to provide given file scattered over a disk vol 5 . an OS buffer manager with the re ume Hence the next logical block in. quired features none currently ex a file is not necessarily physically 4 Scheduling Process. ists at least to our knowledge De close to the previous one Since a Management and Interprocess. signing such a facility with prefetch DBMS does considerable sequential Communication. advice block management advice access the result is considerable disk Often the simplest way to orga . and selected force out would be an arm movement nize a multiuser database system is. interesting exercise It would be of The desired service is for blocks to have one OS process per user i e . interest in the context of both a to be stored physically contiguous each concurrent database user runs. paged virtual memory and an ordi and a whole collection to be read in a separate process It is hoped that. nary file system when sequential access is desired all users will share the same copy of. The strategy used by most This naturally leads a DBMS to pre the code segment o f the database. DBMSs for example System R 4 fer a so called extent based file sys system and perhaps one or more data. and IMS 8 is to maintain a sepa tem e g VSAM 11 to one which segments In particular a DBMS. rate cache in user space This buffer scatters blocks O f course such files buffer pool and lock table should be. pool is managed by a DBMS specific must grow an extent at a time rather handled as a shared segment The. algorithm to circumvent the prob than a block at a time above structure is followed by Sys . lems mentioned in this section The tem R and in part by I N G R E S . result is a not quite right service 3 2 Tree Structured File Systems Since U N I X has no shared data seg . provided by the OS going unused U N I X implements two services ments I N G R E S must put the lock. and a comparable application spe by means of data structures which table inside the operating system and. cific service being provided by the are trees The blocks in a given file provide buffering private to each. DBMS Throughout this paper we are kept track of in a tree of indirect user . will see variations on this theme in blocks pointed to by a file control The alternative organization is to. several service delivery areas block node Second the files in a allocate one run time database pro . given mounted file system have a cess which acts as a server A l l c o n . 3 The File System user visible hierarchical structure current users send messages to this. The file system provided by composed o f directories subdirecto server with work requests The one. U N I X supports objects files which ties etc This is implemented by a run time server schedules requests. are character arrays o f dynamically second tree A DBMS such as through its own mechanisms and. varying size On top of this abstrac I N G R E S then adds a third tree may support its own multitasking. tion a DBMS can provide whatever structure to support keyed access via system This organization is followed. higher level objects it wishes a multilevel directory structure e g by Enscribe 21 Figure 2 shows. This is one o f two popular ap ISAM 7 B trees 1 12 VSAM both possibilities . proaches to file systems the second 11 etc Although Lauer 14 points out. is to provide a record management Clearly one tree with all three that the two methods are equally. system inside the OS e g RMS 11 kinds of information is more efficient viable in a conceptual sense the de . for DEC machines or Enscribe for than three separately managed trees sign o f most operating systems. T a n d e m machines In this approach The extra overhead for three sepa strongly favors the first approach . structured files are provided with or rate trees is probably substantial For example U N I X contains a mes . without variable length records sage system pipes which is incom . Moreover efficient access is often 3 3 Summary, patible with the notion of a server. supported for fetching records cor It is clear that a character array process Hence it forces the use of. responding to a user supplied value is not a useful object to a DBMS the first alternative There are at least. or key for a designated field or Rather it is the abstraction presum two problems with the process per . fields Multilevel directories hash ably desired by language processors user approach . ing and secondary indexes are often editors etc Instead of providing rec . used to provide this service ords management on top o f character 4 1 Performance. The point to be made in this sec arrays it is possible to do the con Every time a run time database. tion is that the second service which verse the only issue is one o f effi process issues an I O request that. is what a DBMS wants is not always ciency Moreover editors can possi cannot be satisfied by data in the. efficient when constructed on top o f bly use records management struc buffer pool a task switch is inevita . 414 Communications July 1981, of Volume 24, the A C M Number 7. To achieve internal parallelism, user 1 user k user 1 0 0 0 user k yet avoid multitasking one could. 000 have user processes send work re , quests to one of perhaps several com . mon servers as noted in Figure 3 , DBMS I DBMS DBMS I However such servers would have to.
process process process share a lock table and are only. slightly different from the shared, code process per user model Alter . nately one could have a collection, of servers each of which would send. low level requests to a group of disk, processes which actually peform the. I O and handle locking as suggested, in Figure 4 A disk process would. process requests in first in first out, Process Per User Server DBMS order Although this organization.
Structure Structure appears potentially desirable it still. Fig 2 Two Approachesto Organizing a Multiuser Database System may have the response time penalty. mentioned above Moreover it re , suits in one message per I O request . In reality one has traded a task, ble The DBMS suspends while wait As a result of these two problems. switch per I 0 for a message per, ing for required data and another with the process per user model one. I O the latter may turn out to be, process is run It is possible to make might expect the server model to be. more expensive than the former In, task switches very efficiently and especially attractive The following.
the next subsection we discuss mes , some operating systems can perform subsection explores this point of. sage costs in more detail ,a task switch in a few hundred in view . structions However many operating 4 4 Performance of Message. systems have large processes i e 4 3 The Server Model Systems. ones with a great deal of state infor A server model becomes viable if Although we have never been of . mation e g accounting and a so the operating system provides a mes fered a good explanation o f why. phisticated scheduler This tends to sage facility which allows n processes messages are so expensive the fact. cause task switches costing a thou to originate messages to a single des remains that in most operating sys . sand instructions or more This is a tination process However such a tems the cost for a round trip mes . high price to pay for a buffer pool server must do its own scheduling sage is several thousand instructions . miss and multitasking This involves a For example in PDP 11 70 U N I X. painful duplication of operating sys the number is about 5 000 As a re . 4 2 Critical Sections tem facilities In order to avoid such suit care must be exercised in a. Blasgen 3 has pointed out that duplication one must resort to the DBMS to avoid overuse o f a facility. some DBMS processes have critical following tactics that is not cheap Consequently vi . sections If the buffer pool is a shared One can avoid multitasking and able DBMS organizations will some . data segment then portions o f the a scheduler by a first come first times be rejected because of exces . Operating System Support for Database Management Michael Stonebraker University of California Berkeley 1 Introduction Database management systems DBMS provide higher level user support than conventional operating systems The DBMS designer must work in the context of the OS he she is faced with Different operating

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