Lec13_FS - File Systems CSE120 Principles of Operating...

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1 CSE120 Principles of Operating Systems Prof Yuanyuan (YY) Zhou Lecture13: I/O & File Systems ov 16 010 Nov 16, 2010 File Systems First we’ll discuss properties of physical disks tructure Structure Performance Scheduling Then we’ll discuss how we build file systems on them Files Directories haring 11/15/2010 CSE 120 2 Sharing Protection File System Layouts File Buffer Cache Read Ahead Stages of I/O Request 11/15/2010 CSE 120 3 Physical Disk Structure Disk components Platters Surfaces Tracks Sectors Cylinders Arm Heads 11/15/2010 CSE 120 4
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2 Another View of Disk 11/15/2010 CSE 120 5 Disk Interaction Specifying disk requests requires a lot of info: Cylinder #, surface #, track #, sector #, transfer size… y Older disks required the OS to specify all of this The OS needed to know all disk parameters Modern disks are more complicated Not all sectors are the same size, sectors are remapped, etc. urrent disks provide a higher- vel interface (SCSI) 11/15/2010 CSE 120 6 Current disks provide a higher level interface (SCSI) The disk exports its data as a logical array of blocks [0…N] Disk maps logical blocks to cylinder/surface/track/sector Only need to specify the logical block # to read/write But now the disk parameters are hidden from the OS Disks: 2006 Seagate Cheetah 3.5" ( server ) capacity: 73 - 300 GB tational speed: 15 000 RPM rotational speed: 15,000 RPM sequential read performance: 135 MB/s (outer) - 82 MB/s (inner) seek time (average): 3.8 ms Seagate Barracuda 3.5" ( workstation ) capacity: 250 - 750 GB rotational speed: 7,200 RPM sequential read performance: 78 MB/s - 44 MB/s (inner) seek time (average): 8.1 ms 11/15/2010 CSE 120 7 Seagate Savvio 2.5" ( smaller form factor ) capacity: 73 GB rotational speed: 10,000 RPM sequential read performance: 62 MB/s (outer) - 42 MB/s (inner) seek time (average): 4.3 ms Disk Performance Disk request performance depends upon three steps eek oving the disk arm to the correct cylinder Seek moving the disk arm to the correct cylinder Depends on how fast disk arm can move ( increasing very slowly ) Rotation – waiting for the sector to rotate under the head Depends on rotation rate of disk ( increasing, but slowly ) Transfer – transferring data from surface into disk controller electronics, sending it back to the host Depends on density ( increasing quickly ) 11/15/2010 CSE 120 8 Analogy: Baggage claim at airports When the OS uses the disk, it tries to minimize the cost of all of these steps Particularly seeks and rotation
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3 Disk Scheduling Because seeks are so expensive (milliseconds!), the OS tries to schedule disk requests that are queued waiting for the disk FCFS (do nothing) Reasonable when load is low Long waiting times for long request queues SSTF (shortest seek time first) Minimize arm movement (seek time), maximize request rate Favors middle blocks CAN ( l t ) 11/15/2010 CSE 120 9 SCAN (elevator) Service requests in one direction until done, then reverse C-SCAN
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Lec13_FS - File Systems CSE120 Principles of Operating...

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