B. Ramamurthy
B.Ramamurthy
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 In addition to primary memory (volatile), computer systems provide users secondary storage units which may be used for persistent (or permanent) storage.
 A file is a collection of data elements grouped together for the purposes of access control, retrieval and modification.
 A file system together with IO management subsystem accomplish the mapping of abstract user interface into the actual collection of hardware such as disk, tape etc. of varying characteristics.
 Recently there has been lot of interest in a storage medium called RAID. We will look into this (11.6)
 We will study IO management in this discussion (Ch.11) and File system in the next chapter (Ch. 12)
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 IO devices
 Device Characteristics
 IO Design Objectives
 A model of IO Organization
 Buffering
 Disk IO
 Disk scheduling
 Disk cache
 RAID storage
 Summary
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 IO devices can be grouped into three categories:
 Human readable: Suitable for use with computer user. Example: video display terminals, keyboard, mouse and printers.
 Machines readable: Example: disks, tapes, sensors, controllers.
 Communication: Suitable for communicating with remote devices: Drivers, modems, sockets, etc.
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 Devices vary widely in characteristics such as: Data rate, applications, complexity of control, unit of transfer, data representation, Error conditions.
 Programmed IO , Interrupt-driven IO and
Direct-memory access are three techniques for performing IO.
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 Efficiency : IO operations often form the bottleneck in a computing system.
 Generality: Handle all devices in a uniform manner.
How?
 Use hierarchical, modular approach to design of IO subsystem.
 This approach hides most of the details of device details in lower-levels so that processes and upper levels of OS see devices in terms of general functions such as Read, Write, Open, Lock, UnLock etc.
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User Processes User Processes User Processes
Logical IO
Comm. Arch.
Device IO
Sched &
Control
Device IO
Sched &
Control
Hardware Hardware
Local Peripheral Device Comm Port
Dir. Mgt.
File System
Physical Org.
Device IO
Sched. &
Control
Hardware
File System
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 In order to avoid inefficiencies associated with direct transfer from user-process to IO device, buffering schemes can be used.
 Buffering smoothes out the peaks in IO demand.
 Buffering depends on the type of IO device:
Block-oriented (Ex: disks) and Stream-oriented
(terminal, comm. ports).
 Stream-oriented IO: single buffering;
 Block-oriented Devices: double buffer, circular buffer
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 A disk contains a stack of recording surfaces each surface with a movable read/write head.
 Each surface has circular tracks and each track divided into sectors. A stack of tracks make up a cylinder.
 To read or write, the head must be positioned at the desired track and at the beginning of the desired sector.
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 Seek Time (Ts) is the time it takes to position the head at the desired track.
 Rotational Delay (Tr) is the time it takes to to line up the head at the beginning of the desired sector.
 Access Time (Ta ) = Ts + Tr
 The data transfer time (Tt) is the time to read/write data at the desired position.
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 Among the times Seek time is the most critical. Average access time can be improved by reducing the average
Seek Time.
 Assumption: A queue of requests for each IO device is maintained. At any time a number of requests from various processes are in the queue. Consider a sequence of n-requests for scheduling.
 Example: Tracks: 55, 58, 39, 18, 90, 160, 150, 38
 Using FIFO, SSTF, SCAN, C-SCAN. Table 11.3, Fig.11.7
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 A disk cache is a buffer in main memory for disk sectors.
 Cache contains a copy of the some of the sectors on the disk.
 When a IO request is made for a particular sector, if the data is available on disk cache it is taken. Otherwise it can be brought into the cache from the disk.
 Similar to virtual memory principles. Directory blocks are good candidates for caching.
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 Is a set of disk drives viewed by operating system as a single logical device.
 Data are distributed across the physical drives of an array.
 Redundant disk capacity is used to store parity information, which guarantees data recoverability in case of disk failure.
 Raid levels 0 to 7
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 RAID 0 - striping
 RAID 1 - Mirrored
 RAID 2 - Error correcting using Hamming code
 RAID 3 - bit-interleaved parity
 RAID 4 - Block-interleaved parity (independent access)
 RAID 5 - Block-interleaved parity (independent access)
 RAID 6 - P + Q redundancy
 RAID 7 - heterogeneous devices
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 Unique contribution of the RAID is to address the need for redundancy.
 Principle : redundancy + compensate for reduced reliability
 Even though use of multiple heads and actuators result in higher transfer rates, they also introduce higher probability of failure.
 To compensate for this decreased reliability,
RAID makes use of stored parity information to recover data lost due to disk failure.
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 RAID 0 : data is “striped” across many disks similar to interleaved memory.
 Not actually RAID.
 No redundancy but high transfer rate due to organization.
 RAID 1 : redundancy is achieved by simple duplicating.
 Read can be performed by either disk which ever has lower seek and rotational time.
 Write can be performed simultaneous on the two disks.
 Recovery from failures is simple: use the surviving disk.
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 RAID 2 - RAID 5 have some kind of parity.
 RAID 2 :
 striping
 Hamming error correcting code
 4 data disks , 3 Hamming code disks
 Effective in environment which are error prone.
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 Similar to RAID but only one single redundant disk.
 Single redundant disk contains the parity info of the bits across the four disks.
 Single disk failure can be tolerated with slightly lower transfer rate until the failed disk is replaced.
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 Independently accessible.
 Strip parity (text calls it block parity).
 Large strips.
 In RAID 4 parity is stored on a single disk, and in RAID 5 parity is distributed over disks.
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 RAID 6 : real fault tolerance hardware included. Significantly more expensive than RAID 5.
 RAID 7 : using distributed and heterogenous sources of data.
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 Look at exercises 11.1, 11.2, 11.7, 11.8
 Hardware characteristics underlying IO devices are abstracted for presenting easier-to-use and uniform logical interface to the processes.
 Performance improvement is another important objective which is achieved through buffering schemes and scheduling algorithms.
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