SECTIONS 13.1 – 13.3 Sanuja Dabade & Eilbroun Benjamin CS 257 – Dr. TY Lin SECONDARY STORAGE MANAGEMENT

Presentation Outline  13.1 The Memory Hierarchy  The Memory Hierarchy  Transfer of Data Between Levels  Volatile and Nonvolatile Storage  Virtual Memory  13.2 Disks  Mechanics of Disks  The Disk Controller  Disk Access Characteristics

Presentation Outline (con’t)  13.3 Accelerating Access to Secondary Storage  The I/O Model of Computation  Organizing Data by Cylinders  Using Multiple Disks  Mirroring Disks  Disk Scheduling and the Elevator Algorithm  Prefetching and Large-Scale Buffering

Memory Hierarchy  Cost per byte to store data also varies  Device with smallest capacity offer the fastest speed with highest cost per bit  These components have data capacities ranging over at least seven orders of magnitude and also have access speeds ranging over seven or more orders of magnitude.

Memory Hierarchy Diagram Programs, DBMS Main Memory DBMS’s Main Memory Cache As Visual Memory Disk File System Tertiary Storage

Memory Hierarchy  Cache  Lowest level of the hierarchy  Data items are copies of certain locations of main memory  Sometimes, values in cache are changed and corresponding changes to main memory are delayed  Machine looks for instructions as well as data for those instructions in the cache  Holds limited amount of data  Cache data can be accessed by the processor in a few nanoseconds.

Memory Hierarchy (con’t)  No need to update the data in main memory immediately in a single processor computer  In multiple processors data is updated immediately to main memory….called as write through

Main Memory  Everything happens in the computer i.e. instruction execution, data manipulation, as working on information that is resident in main memory  Main memories are random access….one can obtain any byte in the same amount of time

Secondary storage  Used to store data and programs when they are not being processed  More permanent than main memory, as data and programs are retained when the power is turned off  E.g. magnetic disks, hard disks

Tertiary Storage  Holds data volumes in terabytes  Used for databases much larger than what can be stored on disk

Transfer of Data Between levels  Data moves between adjacent levels of the hierarchy  At the secondary or tertiary levels accessing the desired data or finding the desired place to store the data takes a lot of time  Disk is organized into bocks  Entire blocks are moved to and from memory called a buffer

Transfer of Data Between level (cont’d)  A key technique for speeding up database operations is to arrange the data so that when one piece of data block is needed it is likely that other data on the same block will be needed at the same time  Same idea applies to other hierarchy levels

Volatile and Non Volatile Storage  A volatile device forgets what data is stored on it after power off  Non volatile holds data for longer period even when device is turned off  All the secondary and tertiary devices are non volatile and main memory is volatile  Magnetic and optical materials hold their data in the absence of power. Thus, essentially all secondary and tertiary storage devices are nonvolatile. On the other hand main memory is generally volatile.

Virtual Memory  Typical software executes in virtual memory  Address space is typically 32 bit or 2 32 bytes or 4GB  Transfer between memory and disk is in terms of blocks

Mechanism of Disk  Mechanisms of Disks  Use of secondary storage is one of the important characteristic of DBMS  Consists of 2 moving pieces of a disk 1. disk assembly 2. head assembly  Disk assembly consists of 1 or more platters  Platters rotate around a central spindle  Bits are stored on upper and lower surfaces of platters

Mechanism of Disk  Disk is organized into tracks  The track that are at fixed radius from center form one cylinder  Tracks are organized into sectors  Tracks are the segments of circle separated by gap

Disk Controller  One or more disks are controlled by disk controllers  Disks controllers are capable of  Controlling the mechanical actuator that moves the head assembly  Selecting the sector from among all those in the cylinder at which heads are positioned  Transferring bits between desired sector and main memory  Possible buffering an entire track

Disk Access Characteristics  Accessing (reading/writing) a block requires 3 steps  Disk controller positions the head assembly at the cylinder containing the track on which the block is located. It is a ‘seek time’  The disk controller waits while the first sector of the block moves under the head. This is a ‘rotational latency’  All the sectors and the gaps between them pass the head, while disk controller reads or writes data in these sectors. This is a ‘transfer time’

Questions