CORNELL CS 432 - Storing Data: Disks and Files

Unformatted text preview:

Database Management System, R. Ramakrishnan and J. Gehrke 1Storing Data: Disks and FilesDatabase Management System, R. Ramakrishnan and J. Gehrke 2Storing and Retrieving Data Database Management Systems need to:– Store large volumes of data– Store data reliably (so that data is not lost!)– Retrieve data efficiently Alternatives for storage– Main memory–Disks–TapeDatabase Management System, R. Ramakrishnan and J. Gehrke 3Why Not Store Everything in Main Memory? Costs too much. $500 will buy you either 512MB of RAM or 100GB of disk today. Main memory is volatile. We want data to be saved between runs. (Obviously!)Database Management System, R. Ramakrishnan and J. Gehrke 4Why Not Store Everything in Tapes? No random access. Data has to be accessed sequentially– Not a great idea when accessing a small portion of a terabyte of data Slow! Data access times are larger than for disksDatabase Management System, R. Ramakrishnan and J. Gehrke 5Disks Secondary storage device of choice–Cheap– Stable storage medium– Random access to data Main problem– Data read/write times much larger than for main memoryDatabase Management System, R. Ramakrishnan and J. Gehrke 6Solution 1: Techniques for makingdisks faster Intelligent data layout on disk– Put related data items together Redundant Array of Inexpensive Disks (RAID)– Achieve parallelism by using many disksDatabase Management System, R. Ramakrishnan and J. Gehrke 7Solution 2: Buffer Management  Keep “currently used” data in main memory– How do we do this efficiently? Typical storage hierarchy:– Main memory (RAM) for currently used data– Disks for the main database (secondary storage)– Tapes for archiving older versions of the data (tertiary storage)Database Management System, R. Ramakrishnan and J. Gehrke 8Outline  Disk technology and how to make disk read/writes faster Buffer management Storing “database files” on diskDatabase Management System, R. Ramakrishnan and J. Gehrke 9Components of a Disk Platters The platters spin (say, 100rps).Spindle The arm assembly is moved in or out to position a head on a desired track. Tracks under heads make a cylinder (imaginary!).Disk headArm movementArm assembly Only one head reads/writes at any one time.TracksSector Block size is a multiple of sector size (which is fixed).Database Management System, R. Ramakrishnan and J. Gehrke 10Accessing 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 1 to 20msec– Rotational delay varies from 0 to 10msec– Transfer rate is about 0.5msec per 4KB page Key to lower I/O cost: reduce seek/rotation delays! Hardware vs. software solutions?Database Management System, R. Ramakrishnan and J. Gehrke 11Arranging 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!Database Management System, R. Ramakrishnan and J. Gehrke 12RAID Disk Array: Arrangement of several disks that gives abstraction of a single, large disk. Goals: Increase performance and reliability.  Two main techniques:– Data striping: Data is partitioned; size of a partition is called the striping unit. Partitions are distributed over several disks.– Redundancy: More disks -> more failures. Redundant information allows reconstruction of data if a disk fails.Database Management System, R. Ramakrishnan and J. Gehrke 13RAID Levels Level 0: No redundancy Level 1: Mirrored (two identical copies)– Each disk has a mirror image (check disk)– Parallel reads, a write involves two disks.– Maximum transfer rate = transfer rate of one disk Level 0+1: Striping and Mirroring– Parallel reads, a write involves two disks.– Maximum transfer rate = aggregate bandwidthDatabase Management System, R. Ramakrishnan and J. Gehrke 14RAID Levels (Contd.) Level 3: Bit-Interleaved Parity– Striping Unit: One bit. One check disk.– Each read and write request involves all disks; disk array can process one request at a time. Level 4: Block-Interleaved Parity– Striping Unit: One disk block. One check disk.– Parallel reads possible for small requests, large requests can utilize full bandwidth– Writes involve modified block and check disk Level 5: Block-Interleaved Distributed Parity– Similar to RAID Level 4, but parity blocks are distributed over all disksDatabase Management System, R. Ramakrishnan and J. Gehrke 15Disk Space Management Lowest layer of DBMS software manages space on disk. Higher levels call upon this layer to:– allocate/de-allocate a page– read/write a page Request for a sequence of pages must be satisfied by allocating the pages sequentially on disk! Higher levels don’t need to know how this is done, or how free space is managed.Database Management System, R. Ramakrishnan and J. Gehrke 16Outline  Disk technology and how to make disk read/writes faster Buffer management Storing “database files” on diskDatabase Management System, R. Ramakrishnan and J. Gehrke 17Buffer Management in a DBMS Data must be in RAM for DBMS to operate on it! Table of <frame#, pageid> pairs is maintained.DBMAIN MEMORYDISKdisk pagefree framePage Requests from Higher LevelsBUFFER POOLchoice of frame dictatedby replacement policyDatabase Management System, R. Ramakrishnan and J. Gehrke 18When 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-fetchedseveral pages at a time!Database Management System, R. Ramakrishnan and J. Gehrke 19More 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


View Full Document

CORNELL CS 432 - Storing Data: Disks and Files

Download Storing Data: Disks and Files
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Storing Data: Disks and Files and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Storing Data: Disks and Files 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?