A+ Guide to Managing and Maintaining your PC, 6e2 RAID Controllers Redundant Array of Independent (or Inexpensive) Disks Level 0 -- Striped Disk Array without Fault Tolerance: Provides data striping (spreading out blocks of each file across multiple disk drives) but no redundancy. This improves performance but does not deliver fault tolerance. If one drive fails then all data in the array is lost. Level 1 -- Mirroring and Duplexing: Provides disk mirroring. Level 1 provides twice the read transaction rate of single disks and the same write transaction rate as single disks. Level 2 -- Error-Correcting Coding: Not a typical implementation and rarely used, Level 2 stripes data at the bit level rather than the block level.
A+ Guide to Managing and Maintaining your PC, 6e3 Level 4 -- Dedicated Parity Drive: A commonly used implementation of RAID, Level 4 provides block-level striping (like Level 0) with a parity disk. If a data disk fails, the parity data is used to create a replacement disk. A disadvantage to Level 4 is that the parity disk can create write bottlenecks. Level 5 -- Block Interleaved Distributed Parity: Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance. Level 5 is one of the most popular implementations of RAID. Level 6 -- Independent Data Disks with Double Parity: Provides block-level striping with parity data distributed across all disks. Level 0+1 – A Mirror of Stripes: Not one of the original RAID levels, two RAID 0 stripes are created, and a RAID 1 mirror is created over them. Used for both replicating and sharing data among disks. Level 10 – A Stripe of Mirrors: Not one of the original RAID levels, multiple RAID 1 mirrors are created, and a RAID 0 stripe is created over these. Level 7: A trademark of Storage Computer Corporation that adds caching to Levels 3 or 4. RAID S: EMC Corporation's proprietary striped parity RAID system used in its Symmetrix storage systems.RAID system
A+ Guide to Managing and Maintaining your PC, 6e4 RAID 0 & 1 http://www.adtron.com/expertise/activeraid.html http://www.adtron.com/expertise/activeraid.html
A+ Guide to Managing and Maintaining your PC, 6e5 RAID 5 RAID 5 ensures that if one of the disks in the striped set fails, its contents can be extracted using the information on the remaining functioning disks.
A+ Guide to Managing and Maintaining your PC, 6e6 RAID 10 Striping + Mirrors, improves performance and give redundancy.
Table 11.4 RAID Levels N = number of data disks; m proportional to log N
Files Data collections created by users The File System is one of the most important parts of the OS to a user Desirable properties of files:
File Structure Provide a means to store data organized as files as well as a collection of functions that can be performed on files Maintain a set of attributes associated with the file Typical operations include: Create Delete Open Close Read Write
Structure Terms Field –basic element of data –contains a single value –fixed or variable length File collection of related fields that can be treated as a unit by some application program fixed or variable length Record Database collection of similar records treated as a single entity may be referenced by name access control restrictions usually apply at the file level collection of related data relationships among elements of data are explicit designed for use by a number of different applications consists of one or more types of files
File Management System Objectives Meet the data management needs of the user Guarantee that the data in the file are valid Optimize performance Provide I/O support for a variety of storage device types Minimize the potential for lost or destroyed data Provide a standardized set of I/O interface routines to user processes Provide I/O support for multiple users in the case of multiple- user systems
The Pile Least complicated form of file organization Data are collected in the order they arrive Each record consists of one burst of data Purpose is simply to accumulate the mass of data and save it Record access is by exhaustive search
The Sequential File Most common form of file structure A fixed format is used for records Key field uniquely identifies the record Typically used in batch applications Only organization that is easily stored on tape as well as disk
Indexed Sequential File Adds an index to the file to support random access Adds an overflow file Greatly reduces the time required to access a single record Multiple levels of indexing can be used to provide greater efficiency in access
Indexed File Records are accessed only through their indexes Variable-length records can be employed Exhaustive index contains one entry for every record in the main file Partial index contains entries to records where the field of interest exists Used mostly in applications where timeliness of information is critical Examples would be airline reservation systems and inventory control systems
Direct or Hashed File Access directly any block of a known address Makes use of hashing on the key value Often used where: very rapid access is required fixed-length records are used records are always accessed one at a time
B-Trees A balanced tree structure with all branches of equal length Standard method of organizing indexes for databases Commonly used in OS file systems Provides for efficient searching, adding, and deleting of items
A B-tree is characterized by its minimum degree d and satisfies the following properties: every node has at most 2 d – 1 keys and 2 d children or, equivalently, 2 d pointers every node, except for the root, has at least d – 1 keys and d pointers, as a result, each internal node, except the root, is at least half full and has at least d children the root has at least 1 key and 2 children all leaves appear on the same level and contain no information. This is a logical construct to terminate the tree; the actual implementation may differ. a nonleaf node with k pointers contains k – 1 keys