Mass storage structure
- 2. Disk drive structure Data stored on surfaces Up to two surfaces per platter One or more platters per disk Data in concentric tracks Tracks broken into sectors 256B-1KB per sector Cylinder: corresponding tracks on all surfaces Data read and written by heads Actuator moves heads Heads move in unison Chapter 5 2 CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and sector cylinder platter spindle track head actuator surfaces
- 3. Disk drive specifics IBM 360KB floppy WD 18GB HD Cylinders 40 10601 Tracks per cylinder 2 12 Sectors per track 9 281 (average) Sectors per disk 720 35742000 Bytes per sector 512 512 Capacity 360 KB 18.3 GB Seek time (minimum) 6 ms 0.8 ms Seek time (average) 77 ms 6.9 ms Rotation time 200 ms 8.33 ms Spinup time 250 ms 20 sec Sector transfer time 22 ms 17 msec Chapter 5 3 CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and
- 4. Disk “zones” Outside tracks are longer than inside tracks Two options Bits are “bigger” More bits (transfer faster) Modern hard drives use second option More data on outer tracks Disk divided into “zones” Constant sectors per track in each zone 8–20 (or more) zones on a disk Chapter 5 4 CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and
- 5. Disk “addressing” Millions of sectors on the disk must be labeled Two possibilities Cylinder/track/sector Sequential numbering Modern drives use sequential numbers Disks map sequential numbers into specific location Mapping may be modified by the disk Remap bad sectors Optimize performance Hide the exact geometry, making life simpler for the OS CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt) Chapter 5 5
- 6. Building a better “disk” Problem: CPU performance has been increasing exponentially, but disk performance hasn’t Disks are limited by mechanics Problem: disks aren’t all that reliable Solution: distribute data across disks, and use some of the space to improve reliability Data transferred in parallel Data stored across drives (striping) Parity on an “extra” drive for reliability CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt) Chapter 5 6
- 7. CD-ROM recording CD-ROM has data in a spiral Hard drives have concentric circles of data One continuous track: just like vinyl records! Pits & lands “simulated” with heat-sensitive material on CD-Rs and CD-RWs Chapter 5 7 CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and
- 8. Structure of a disk sector Preamble contains information about the sector Sector number & location information Data is usually 256, 512, or 1024 bytes ECC (Error Correcting Code) is used to detect & correct minor errors in the data Chapter 5 8 CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Preamble Data ECC
- 9. What’s in a disk request? Time required to read or write a disk block determined by 3 factors Seek time Rotational delay Average delay = 1/2 rotation time Example: rotate in 10ms, average rotation delay = 5ms Actual transfer time Transfer time = time to rotate over sector Example: rotate in 10ms, 200 sectors/track => 10/200 ms = 0.05ms transfer time per sector Seek time dominates, with rotation time close Error checking is done by controllers CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt) Chapter 5 9
- 10. Disk request scheduling Goal: use disk hardware efficiently Bandwidth as high as possible Disk transferring as often as possible (and not seeking) We want to Minimize disk seek time (moving from track to track) Minimize rotational latency (waiting for disk to rotate the desired sector under the read/write head) Calculate disk bandwidth by Total bytes transferred / time to service request Seek time & rotational latency are overhead (no data is transferred), and reduce disk bandwidth Minimize seek time & rotational latency by Using algorithms to find a good sequence for servicing requests Placing blocks of a given file “near” each other CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt) Chapter 5 10
- 11. Disk scheduling algorithms Schedule disk requests to minimize disk seek time Seek time increases as distance increases (though not linearly) Minimize seek distance -> minimize seek time Disk seek algorithm examples assume a request queue & head position (disk has 200 cylinders) Queue = 100, 175, 51, 133, 8, 140, 73, 77 Head position = 63 Chapter 5 11 CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and 100 17551 1338 140 73 77 read/write head position disk requests (cylinder in which block resides) Outside edge Inside edge
- 12. First-Come-First Served (FCFS) Requests serviced in the order in which they arrived Easy to implement! May involve lots of unnecessary seek distance Seek order = 100, 175, 51, 133, 8, 140, 73, 77 Seek distance = (100-63) + (175-100) + (175-51) + (133-51) + (133-8) + (140-8) + (140-73) + (77-73) = 646 cylinders Chapter 5 12 CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and 100 175 51 133 8 140 73 77 read/write head start
- 13. Shortest Seek Time First (SSTF) Service the request with the shortest seek time from the current head position Form of SJF scheduling May starve some requests Seek order = 73, 77, 51, 8, 100, 133, 140, 175 Seek distance = 10 + 4 + 26 + 43 + 92 + 33 + 7 + 35 = 250 cylinders Chapter 5 13 CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and 100 175 51 133 8 140 73 77 read/write head start
- 14. SCAN (elevator algorithm) Disk arm starts at one end of the disk and moves towards the other end, servicing requests as it goes Reverses direction when it gets to end of the disk Also known as elevator algorithm Seek order = 51, 8, 0 , 73, 77, 100, 133, 140, 175 Seek distance = 12 + 43 + 8 + 73 + 4 + 23 + 33 + 7 + 35 = 238 cyls Chapter 5 14 CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and 100 175 51 133 8 140 73 77 read/write head start
- 15. C-SCAN Identical to SCAN, except head returns to cylinder 0 when it reaches the end of the disk Treats cylinder list as a circular list that wraps around the disk Waiting time is more uniform for cylinders near the edge of the disk Seek order = 73, 77, 100, 133, 140, 175, 199, 0, 8, 51 Distance = 10 + 4 + 23 + 33 + 7 + 35 + 24 + 199 + 8 + 43 = 386 cyls Chapter 5 15 CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and 100 175 51 133 8 140 73 77 read/write head start
- 16. C-LOOK Identical to C-SCAN, except head only travels as far as the last request in each direction Saves seek time from last sector to end of disk Seek order = 73, 77, 100, 133, 140, 175, 8, 51 Distance = 10 + 4 + 23 + 33 + 7 + 35 + 167 + 43 = 322 cylinders Chapter 5 16 CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and 100 175 51 133 8 140 73 77 read/write head start
- 17. Selection of a disk scheduling algorithm SSTF is easy to implement and works OK if there aren’t too many disk requests in the queue SCAN-type algorithms perform better for systems under heavy load More fair than SSTF Use LOOK rather than SCAN algorithms to save time Long seeks aren’t too expensive, so choose C-LOOK over LOOK to make response time more even Disk request scheduling interacts with algorithms for allocating blocks to files Make scheduling algorithm modular: allow it to be changed without changing the file system Use SSTF for lightly loaded systems Use C-LOOK for heavily loaded systems CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt) Chapter 5 17
- 18. Disk Management Low-level formatting, or physical formatting — Dividing a disk into sectors that the disk controller can read and write To use a disk to hold files, the operating system still needs to record its own data structures on the disk Partition the disk into one or more groups of cylinders Logical formatting or “making a file system” To increase efficiency most file systems group blocks into clusters Disk I/O done in blocks File I/O done in clusters Boot block initializes system The bootstrap is stored in ROM Bootstrap loader program Methods such as sector sparing used to handle bad blocks
- 19. Swap-Space Management Swap-space — Virtual memory uses disk space as an extension of main memory. Swap-space can be carved out of the normal file system,or, more commonly, it can be in a separate disk partition. Swap-space management 4.3BSD allocates swap space when process starts; holds text segment (the program) and data segment. Kernel uses swap maps to track swap-space use. Solaris 2 allocates swap space only when a page is forced out of physical memory, not when the virtual memory page is first created.
- 20. Swapping on Linux Linux uses 4K page slots Swap can be in dedicated swap partition or swap file on FS Numbers above indicate ref count; e.g., 3 means 3 processes sharing