Module 4 topic 1 - Module 4, Topic 1 page 1/9 Module 4,...

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Module 4, Topic 1 page 1/9 Module 4, Topic 1 -File Systems and Disks Return to List of Topics. Topics. Reading RAID Levels. Disk mirroring. Disk duplexing. RAID Levels, details. RAID Technology. Software-based RAID. Hardware-based RAID SCSI-to-SCSI RAID. HSM software . LAN-based HSM technology. Self Assessment Exercises Reading Chapter 10 of Systems Administration. RAID Levels. In an effort to provide large amounts of data storage combined with fault tolerance and redundancy, numerous small disk drives are joined together in arrays and controlled by software which then makes these numerous disks appear to a server operating system as one gigantic disk. Exactly how these numerous disks are physically and logically linked is defined by a series of standards known as RAID. RAID originally stood for Redundant Array of Inexpensive Disks, but now often refers to Redundant Array of Independent Disks. RAID is not the first method used to provide fault-tolerant or redundant data storage. Individual network operating systems previously offered two closely related methods for ensuring data availability for mission-critical applications:
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Module 4, Topic 1 page 2/9 1. Disk mirroring. This involves two disks attached to the same controller with each disk acting as a mirror image of each other. Everything written to one disk is identically written to the other. In the event that one disk fails, the other disk immediately takes over. The single point of failure in this scheme is the single disk controller which controls the two mirrored disks. If the single controller fails, both disks become unreachable. 2. Disk duplexing. In order to address the vulnerability of disk mirroring to a failed disk controller, this seeks to overcome this single point of failure by linking a separate disk controller to each mirrored disk drive. RAID Levels. RAID incorporates, but also goes beyond, disk mirroring and disk duplexing, including six different RAID levels. These levels serve as standards for hardware and software vendors selling RAID technology. RAID standards are maintained by the RAID Advisory Board (RAB). Figure 5 - 14 (page 170) lists key facts concerning the RAID levels officially recognized by RAB. The text in the comments column in Figure 5-14, attributed to Joe Molina, the RAID Advisory Board Chairman, are included for insight into the practical, rather than, theoretical, significance of the various RAID levels. Figure 5-15 (page 171) lists other RAID level definitions used by RAID technology vendors but not officially sanctioned by the RAB. RAID level Definition Comment Application Advantages Disadvantages Level 1 Disk mirroring Disk mirroring, also known as shadowing Mirroring System drives, critical files High reliability Writing to both mirrored disks degrades write performance, expensive.
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Module 4, Topic 1 page 3/9 Level 2 Striped Array plus hamming code Writes data across multiple disks, adding hamming code for error
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Module 4 topic 1 - Module 4, Topic 1 page 1/9 Module 4,...

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