Storage in Database Systems. CMPSCI 445 Fall 2010

Transcription

1 Storage in Database Systems CMPSCI 445 Fall

2 Storage Topics Architecture and Overview Disks Buffer management Files of records 2

3 DBMS Architecture Query Parser Query Rewriter Query Optimizer Query Executor Lock Manager File & Access Methods Buffer Manager Log Manager Disk Space Manager 3

4 Data on External Storage Disks: Can retrieve random page at fixed cost But reading several consecutive pages is much cheaper than reading them in random order Tapes: Can only read pages in sequence Cheaper than disks; used for archival storage Page: Unit of information read from or written to disk Size of page: DBMS parameter, 4KB, 8KB Disk space manager: Abstraction: a collection of pages. Allocate/de-allocate a page. Read/write a page. Page I/O: Pages read from disk and pages written to disk Dominant cost of database operations 4

5 Buffer Management Architecture: Data is read into memory for processing Data is written to disk for persistent storage Buffer manager stages pages between external storage and main memory buffer pool. Access method layer makes calls to the buffer manager. 5

6 Access Methods Access methods: routines to manage various disk-based data structures. Files of records Various kinds of indexes File of records: Important abstraction of external storage in a DBMS! Record id (rid) is sufficient to physically locate a record Indexes: Auxiliary data structures Given values in index search key fields, find the record ids of records with those values 6

7 File organizations & access methods Many alternatives exist, each ideal for some situations, and not so good in others: Heap (unordered) files: Suitable when typical access is a file scan retrieving all records. Sorted Files: Best if records must be retrieved in some order, or only a `range of records is needed. Indexes: Data structures to organize records via trees or hashing. Like sorted files, they speed up searches for a subset of records, based on values in certain ( search key ) fields Updates are much faster than in sorted files. 7

8 Disks 8

9 Disks and DBMS Design DBMS stores information on disks. This has major implications for DBMS design! READ: transfer data from disk to main memory (RAM) for data processing. WRITE: transfer data from RAM to disk for persistent storage. Both are high-cost operations, relative to in-memory operations, so must be planned carefully! 9

10 Why Not Store Everything in Main Memory? Main memory is volatile. We want data to be saved between runs. (Obviously!) Costs too much. $100 will buy you either 4GB of RAM or 1TB of disk today. 32-bit addressing limitation bytes can be directly addressed in memory. Number of objects cannot exceed this number. 10

11 Basics of Disks Unit of storage and retrieval: disk block or page. A disk block/page is a contiguous sequence of bytes. Size of a DBMS parameter, 4KB or 8KB. Disks support direct access to a page. Unlike RAM, time to retrieve a page varies! It depends upon the location on disk. Therefore, relative placement of pages on disk has major impact on DBMS performance! 11

12 Components of a Disk Spindle Platters spin (say, 7200rpm). Arm assembly is moved in or out to position a head on a desired track. Only one head reads/ writes at any one time. Tracks under heads make a cylinder (imaginary!). Each track is divided into sectors (whose size is fixed). Block size is a multiple of sector size. Arm assembly Disk head Arm movement Track Sector Platters 12

13 Accessing a Disk Page Time to access (read/write) a disk block: seek time (moving arms to position disk head on track) rotational delay (waiting for block to rotate under head) transfer time (actually moving data to/from disk surface) Seek time and rotational delay dominate. Seek time varies from about 2 to 30 msec Rotational delay varies from 0 to 11 msec Transfer rate between 25 to 100 MB/s Key to lower I/O cost: reduce seek/rotation delays! 13

14 Arranging Pages on Disk `Next block concept: blocks on same track, followed by blocks on same cylinder, followed by blocks on adjacent cylinder Blocks in a file should be arranged sequentially on disk (by `next ), to minimize seek and rotational delay. For a sequential scan, pre-fetching several pages at a time is a big win! 14

15 Buffer Management 15

16 Buffer Management in a DBMS Page Requests from Higher Levels BUFFER POOL disk page free frame MAIN MEMORY DISK DB choice of frame dictated by replacement policy Data must be in RAM for DBMS to operate on it! Table of <frame#, pageid> pairs is maintained. 16

17 More on Buffer Management Requestor of page must unpin it, and indicate whether page has been modified: dirty bit is used for this. Page in pool may be requested many times, a pin count is used. A page is a candidate for replacement iff pin count = 0. CC & recovery may entail additional I/O when a frame is chosen for replacement. (Write-Ahead Log protocol; more later.) 17

18 When a Page is Requested... If requested page is not in pool: Choose a frame for replacement If frame is dirty, write it to disk Read requested page into chosen frame Pin the page and return its address. If requests can be predicted (e.g., sequential scans) pages can be pre-fetched several pages at a time! 18

19 Buffer Replacement Policy Frame is chosen for replacement by a replacement policy: Least-recently-used (LRU), Clock, MRU etc. Policy can have big impact on # of I/O s; depends on the access pattern. Sequential flooding: Nasty situation caused by LRU + repeated sequential scans. # buffer frames < # pages in file means each page request causes an I/O. MRU much better in this situation (but not in all situations, of course). 19

20 DBMS vs. OS File System OS does disk space & buffer mgmt: why not let OS manage these tasks? Differences in OS support: portability issues Some limitations, e.g., files can t span disks. Buffer management in DBMS requires ability to: pin a page in buffer pool, force a page to disk (important for implementing CC & recovery), adjust replacement policy, and pre-fetch pages based on access patterns in typical DB operations. 20

21 Files 21

22 Files Access method layer offers an abstraction of data on disk: a file of records residing on multiple pages A number of fields are organized in a record A collection of records are organized in a page A collection of pages are organized in a file 22

23 Files of Records Page or block is OK when doing I/O, but higher levels of DBMS operate on records and files of records. FILE: A collection of pages, each containing a collection of records. Must support: insert/delete/modify record read a particular record (specified using record id) scan all records (possibly with some conditions on the records to be retrieved) 23

24 Unordered (Heap) Files Simplest file structure contains records in no particular order. As file grows and shrinks, disk pages are allocated and de-allocated. To support record level operations, we must: keep track of the pages in a file keep track of free space on pages keep track of the records on a page There are many alternatives for keeping track of this. 24

25 Heap File Implemented as a List Data Page Data Page Data Page Full Pages Header Page Data Page Data Page Data Page Pages with Free Space (heap file name, header page id) stored in a known place. Two doubly linked lists, for full pages & pages with space. Each page contains 2 `pointers plus data. Upon insertion, scan the list of pages with space, or ask disk space manager to allocate a new page 25

26 Heap File Using a Page Directory Header Page Data Page 1 Data Page 2 DIRECTORY Data Page N A directory entry per page; it can include the number of free bytes on the page. The directory is a collection of pages; linked list implementation is just one alternative. Much smaller than linked list of all HF pages! 26

27 Page Format How to store a collection of records on a page? Consider a page as a collection of slots, one for each record. A record is identified by rid = <page id, slot #> Record ids (rids) are used in indexes (Alternatives 2 and 3). 27

28 Page Format: Fixed Length Records Slot 1 Slot 2 Slot 1 Slot 2 Slot N... Free Space Slot N... Slot M PACKED N number of records M... M UNPACKED, BITMAP number of slots Moving records for free space management changes rid! May not be acceptable. 28

29 Page Format: Variable Length Records Rid = (i,n) Page i Rid = (i,2) Rid = (i,1) Rid of record is <page id, slot #> N Pointer N to start # slots of free space SLOT DIRECTORY Can move records on page without changing rid; so, attractive for fixed-length records too. (level of indirection) 29

30 Record Format: Fixed Length F1 F2 F3 F4 L1 L2 L3 L4 Base address (B) Address = B+L1+L2 Information of a record type e.g., the number of fields and field types is stored in the system catalog. Fixed length record: (1) the number of fields is fixed, (2) each field has a fixed length. Store fields consecutively in a record. Finding i th field does not require scan of record. 30

31 Record Format: Variable Length Variable length record: (1) number of fields is fixed, (2) some fields are variable length Two alternatives: Fields Delimited by Special Symbols Scan F1 F2 F3 F4 $ $ $ $ Array of Field Offsets S1 S2 S3 S4 E4 F1 F2 F3 F4 Second offers direct access to i th field, efficient storage of NULLs ; small directory overhead. 31