Mass Storage Systems - Operating Systems - Lecture Slides, Slides of Computer Science

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Mass-Storage Systems

Chapter 12

Objectives

• Describe the physical structure of secondary

and tertiary storage devices and the resulting

effects on the uses of the devices

• Explain the performance characteristics of

mass-storage devices

• Discuss operating-system services provided for

mass storage, including RAID and HSM

Moving-head Disk Machanism

Overview of Mass Storage Structure (Cont.)

• Magnetic tape
  • Was early secondary-storage medium
  • Relatively permanent and holds large quantities of data
  • Access time slow
  • Random access ~1000 times slower than disk
  • Mainly used for backup, storage of infrequently-used data, transfer medium between systems
  • Kept in spool and wound or rewound past read-write head
  • Once data under head, transfer rates comparable to disk
  • 20-200GB typical storage
  • Common technologies are 4mm, 8mm, 19mm, LTO-2 and SDLT

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

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

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.

Disk Scheduling (Cont.)

• Several algorithms exist to schedule the

servicing of disk I/O requests.

• We illustrate them with a request queue (0-

Head pointer 53

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.

SSTF (Cont.)

SCAN (Cont.)

C-SCAN

• Provides a more uniform wait time than SCAN.
• The head moves from one end of the disk to the

other. servicing requests as it goes. When it

reaches the other end, however, it immediately

returns to the beginning of the disk, without

servicing any requests on the return trip.

• Treats the cylinders as a circular list that wraps

around from the last cylinder to the first one.

C-LOOK

• Version of C-SCAN

• Arm only goes as far as the last request in

each direction, then reverses direction

immediately, without first going all the way to

the end of the disk.

C-LOOK (Cont.)