Storing Data: Disks and FilesDisks and FilesWhy Not Store Everything in Main Memory?DisksComponents of a DiskAccessing a Disk PageArranging Pages on DiskRAIDRAID LevelsRAID Levels (Contd.)Disk Space ManagementBuffer Management in a DBMSWhen a Page is Requested ...More on Buffer ManagementBuffer Replacement PolicyDBMS vs. OS File SystemRecord Formats: Fixed LengthRecord Formats: Variable LengthPage Formats: Fixed Length RecordsPage Formats: Variable Length RecordsFiles of RecordsUnordered (Heap) FilesHeap File Implemented as a ListHeap File Using a Page DirectoryIndexesSystem CatalogsAttr_Cat(attr_name, rel_name, type, position)SummarySummary (Contd.)Slide 30Database Management Systems, R. Ramakrishnan and J. Gehrke 1Storing Data: Disks and FilesChapter 7“Yea, from the table of my memoryI’ll wipe away all trivial fond records.”-- Shakespeare, HamletDatabase Management Systems, R. Ramakrishnan and J. Gehrke 2Disks and Files DBMS stores information on (“hard”) disks.This has major implications for DBMS design!–READ: transfer data from disk to main memory (RAM).–WRITE: transfer data from RAM to disk.–Both are high-cost operations, relative to in-memory operations, so must be planned carefully!Database Management Systems, R. Ramakrishnan and J. Gehrke 3Why Not Store Everything in Main Memory?Costs too much. $1000 will buy you either 128MB of RAM or 7.5GB of disk today.Main memory is volatile. We want data to be saved between runs. (Obviously!)Typical storage hierarchy:–Main memory (RAM) for currently used data.–Disk for the main database (secondary storage).–Tapes for archiving older versions of the data (tertiary storage).Database Management Systems, R. Ramakrishnan and J. Gehrke 4DisksSecondary storage device of choice. Main advantage over tapes: random access vs. sequential.Data is stored and retrieved in units called disk blocks or pages.Unlike RAM, time to retrieve a disk page varies depending upon location on disk. –Therefore, relative placement of pages on disk has major impact on DBMS performance!Database Management Systems, R. Ramakrishnan and J. Gehrke 5Components of a Disk Platters The platters spin (say, 90rps).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 Systems, R. Ramakrishnan and J. Gehrke 6Accessing 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 1msec per 4KB pageKey to lower I/O cost: reduce seek/rotation delays! Hardware vs. software solutions?Database Management Systems, R. Ramakrishnan and J. Gehrke 7Arranging 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 Systems, R. Ramakrishnan and J. Gehrke 8RAID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 Systems, R. Ramakrishnan and J. Gehrke 9RAID 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 Systems, R. Ramakrishnan and J. Gehrke 10RAID 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 Systems, R. Ramakrishnan and J. Gehrke 11Disk 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 Systems, R. Ramakrishnan and J. Gehrke 12Buffer 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 Systems, R. Ramakrishnan and J. Gehrke 13When 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!Database Management Systems, R. Ramakrishnan and J. Gehrke 14More 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;