40 disk structure disk drives are addressed as large

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40 Disk Structure ! Disk drives are addressed as large 1-dimensional arrays of logical blocks , where the logical block is the smallest unit of transfer. ! The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially. " Sector 0 is the first sector of the first track on the outermost cylinder. " Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost. 79 Disk Attachment ! Host-attached storage accessed through I/O ports talking to I/O busses ! SCSI itself is a bus, up to 16 devices on one cable, SCSI initiator requests operation and SCSI targets perform tasks " Each target can have up to 8 logical units (disks attached to device controller ! FC is high-speed serial architecture " Can be switched fabric with 24-bit address space – the basis of storage area networks ( SAN s) in which many hosts attach to many storage units " Can be arbitrated loop ( FC-AL ) of 126 devices 80
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41 Network-Attached Storage ! Network-attached storage (NAS) is storage made available over a network rather than over a local connection (such as a bus) ! NFS and CIFS are common protocols ! Implemented via remote procedure calls (RPCs) between host and storage ! New iSCSI protocol uses IP network to carry the SCSI protocol 81 Storage Area Network ! Common in large storage environments (and becoming more common) ! Multiple hosts attached to multiple storage arrays - flexible 82
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42 Disk Scheduling ! The operating system is responsible for using hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth. ! Access time has two major components " Seek time is the time for the disk are to move the heads to the cylinder containing the desired sector. " Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. ! Minimize seek time ! Seek time seek distance ! Disk bandwidth is the total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer. 83 Disk Scheduling (cont’d) ! Several algorithms exist to schedule the servicing of disk I/O requests. ! We illustrate them with a request queue (0-199). 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53 84
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43 FCFS Illustration shows total head movement of 640 cylinders. 85 SSTF ! Selects the request with the minimum seek time from the current head position. ! SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests. ! Illustration shows total head movement of 236 cylinders. 86
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44 SSTF (cont’d) 87 SCAN ! The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues. ! Sometimes called the elevator algorithm . ! Illustration shows total head movement of 208 cylinders.
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  • Spring '12
  • GwangS.Jung
  • file systems

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