I/O, Disks, and RAID

1. I/O, Disks, and RAID

2. Goals for Today • Review I/O • How does a computer system interact with its environment? • Disks • How does a computer system permanently store data? • Prelim graded! • Discuss and pass back today • RAID • How to make storage both efficient and reliable?

3. The Requirements of I/O • So far in this course: • We have learned how to manage CPU, memory • What about I/O? • Without I/O, computers are useless (disembodied brains?) • But… thousands of devices, each slightly different • How can we standardize the interfaces to these devices? • Devices unreliable: media failures and transmission errors • How can we make them reliable??? • Devices unpredictable and/or slow • How can we manage them if we don’t know what they will do or how they will perform? • Some operational parameters: • Byte/Block • Some devices provide single byte at a time (e.g. keyboard) • Others provide whole blocks (e.g. disks, networks, etc) • Sequential/Random • Some devices must be accessed sequentially (e.g. tape) • Others can be accessed randomly (e.g. disk, cd, etc.) • Polling/Interrupts • Some devices require continual monitoring • Others generate interrupts when they need service

4. Modern I/O Systems

5. Example Device-Transfer Rates (Sun Enterprise 6000) • Device Rates vary over many orders of magnitude • System better be able to handle this wide range • Better not have high overhead/byte for fast devices! • Better not waste time waiting for slow devices

6. The Goal of the I/O Subsystem • Provide Uniform Interfaces, Despite Wide Range of Different Devices • This code works on many different devices: int fd = open(“/dev/something”); for (int i = 0; i < 10; i++) { fprintf(fd,”Count %d\n”,i); } close(fd); • Why? Because code that controls devices (“device driver”) implements standard interface. • We will try to get a flavor for what is involved in actually controlling devices in rest of lecture • Can only scratch surface!

7. Want Standard Interfaces to Devices • Block Devices:e.g.disk drives, tape drives, DVD-ROM • Access blocks of data • Commands include open(), read(), write(), seek() • Raw I/O or file-system access • Memory-mapped file access possible • Character Devices:e.g.keyboards, mice, serial ports, some USB devices • Single characters at a time • Commands include get(), put() • Libraries layered on top allow line editing • Network Devices:e.g.Ethernet, Wireless, Bluetooth • Different enough from block/character to have own interface • Unix and Windows include socket interface • Separates network protocol from network operation • Includes select() functionality • Usage: pipes, FIFOs, streams, queues, mailboxes

8. How Does User Deal with Timing? • Blocking Interface:“Wait” • When request data (e.g. read() system call), put process to sleep until data is ready • When write data (e.g. write() system call), put process to sleep until device is ready for data • Non-blocking Interface:“Don’t Wait” • Returns quickly from read or write request with count of bytes successfully transferred • Read may return nothing, write may write nothing • Asynchronous Interface:“Tell Me Later” • When request data, take pointer to user’s buffer, return immediately; later kernel fills buffer and notifies user • When send data, take pointer to user’s buffer, return immediately; later kernel takes data and notifies user

9. Life Cycle of An I/O Request User Program Kernel I/O Subsystem Device Driver Top Half Device Driver Bottom Half Device Hardware

10. A Kernel I/O Structure

11. Device Drivers • Device Driver:Device-specific code in the kernel that interacts directly with the device hardware • Supports a standard, internal interface • Same kernel I/O system can interact easily with different device drivers • Special device-specific configuration supported with the ioctl() system call • Device Drivers typically divided into two pieces: • Top half: accessed in call path from system calls • Implements a set of standard, cross-device calls like open(), close(), read(), write(), ioctl(), strategy() • This is the kernel’s interface to the device driver • Top half will start I/O to device, may put thread to sleep until finished • Bottom half: run as interrupt routine • Gets input or transfers next block of output • May wake sleeping threads if I/O now complete

12. I/O Device Notifying the OS • The OS needs to know when: • The I/O device has completed an operation • The I/O operation has encountered an error • I/O Interrupt: • Device generates an interrupt whenever it needs service • Pro: handles unpredictable events well • Con: interrupts relatively high overhead • Polling: • OS periodically checks a device-specific status register • I/O device puts completion information in status register • Could use timer to invoke lower half of drivers occasionally • Pro: low overhead • Con: may waste many cycles on polling if infrequent or unpredictable I/O operations • Some devices combine both polling and interrupts • For instance: High-bandwidth network device: • Interrupt for first incoming packet • Poll for following packets until hardware empty

13. Processor Memory Bus Regular Memory CPU Bus Adaptor Bus Adaptor Device Controller Address+ Data Other Devices or Buses Bus Interface Hardware Controller Interrupt Controller Interrupt Request Addressable Memory and/or Queues Registers (port 0x20) read Memory Mapped Region: 0x8f008020 write control status How does the processor actually talk to the device? • CPU interacts with a Controller • Contains a set of registers that can be read and written • May contain memory for request queues or bit-mapped images • Regardless of the complexity of the connections and buses, processor accesses registers in two ways: • I/O instructions:in/out instructions • Example from the Intel architecture: out 0x21,AL • Memory mapped I/O:load/store instructions • Registers/memory appear in physical address space • I/O accomplished with load and store instructions

14. Transfering Data To/From Controller • Programmed I/O: • Each byte transferred via processor in/out or load/store • Pro: Simple hardware, easy to program • Con: Consumes processor cycles proportional to data size • Direct Memory Access: • Give controller access to memory bus • Ask it to transfer data to/from memory directly • Sample interaction with DMA controller (from book):

15. Main components of Intel Chipset: Pentium 4 • Northbridge: • Handles memory • Graphics • Southbridge: I/O • PCI bus • Disk controllers • USB controllers • Audio • Serial I/O • Interrupt controller • Timers

16. The Memory Hierarchy • Each level acts as a cache for the layer below it CPU registers, L1 cache L2 cache primary memory disk storage (secondary memory) random access tape or optical storage (tertiary memory) sequential access

17. Disks

18. What does the disk look like?

19. Some parameters • 2-30 heads (platters * 2) • diameter 14’’ to 2.5’’ • 700-20480 tracks per surface • 16-1600 sectors per track • sector size: • 64-8k bytes • 512 for most PCs • note: inter-sector gaps • capacity: 20M-300G • main adjectives: BIG, slow

20. Disk overheads • To read from disk, we must specify: • cylinder #, surface #, sector #, transfer size, memory address • Transfer time includes: • Seek time: to get to the track • Latency time: to get to the sector and • Transfer time: get bits off the disk Track Sector Rotation Delay Seek Time

21. Modern disks

22. 52 years ago… • On 13th September 1956, IBM 305 RAMAC computer system first to use disk storage • 80000 times more data on the 8GB 1-inch drive in his right hand than on the 24-inch RAMAC one in his left…

23. Smallest write: sector Atomic write = sector Random access: 5ms not on a good curve Sequential access: 200MB/s Cost $.002MB Crash: doesn’t matter (“non-volatile”) Disks vs. Memory • (usually) bytes • byte, word • 50 ns • faster all the time • 200-1000MB/s • $.10MB • contents gone (“volatile”)

24. Disk Structure • Disk drives addressed as 1-dim arrays of logical blocks • the logical block is the smallest unit of transfer • This array mapped sequentially onto disk sectors • Address 0 is 1st sector of 1st track of the outermost cylinder • Addresses incremented within track, then within tracks of the cylinder, then across cylinders, from innermost to outermost • Translation is theoretically possible, but usually difficult • Some sectors might be defective • Number of sectors per track is not a constant

25. Non-uniform #sectors / track • Maintain same data rate with Constant Linear Velocity • Approaches: • Reduce bit density per track for outer layers • Have more sectors per track on the outer layers (virtual geometry)

26. Disk Scheduling • The operating system tries to use hardware efficiently • for disk drives  having fast access time, disk bandwidth • Access time has two major components • Seek time is time to move the heads to the cylinder containing the desired sector • Rotational latency is additional time waiting to rotate the desired sector to the disk head. • Minimize seek time • Seek time  seek distance • Disk bandwidth is total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer.

27. Disk Scheduling (Cont.) • Several scheduling algos exist service disk I/O requests. • We illustrate them with a request queue (0-199). 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53

28. FCFS Illustration shows total head movement of 640 cylinders.

29. SSTF • Selects request with minimum seek time from 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.

30. SSTF (Cont.)

31. SCAN • The disk arm starts at one end of the disk, • moves toward the other end, servicing requests • head movement is reversed when it gets to the other end of disk • servicing continues. • Sometimes called the elevator algorithm. • Illustration shows total head movement of 236 cylinders.

32. SCAN (Cont.)

33. 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 it immediately returns to beginning of the disk • No requests serviced on the return trip. • Treats the cylinders as a circular list • that wraps around from the last cylinder to the first one.

34. C-SCAN (Cont.)

35. C-LOOK • Version of C-SCAN • Arm only goes as far as last request in each direction, • then reverses direction immediately, • without first going all the way to the end of the disk.

36. C-LOOK (Cont.)

37. Selecting a Good Algorithm • SSTF is common and has a natural appeal • SCAN and C-SCAN perform better under heavy load • Performance depends on number and types of requests • Requests for disk service can be influenced by the file-allocation method. • Disk-scheduling algo should be a separate OS module • allowing it to be replaced with a different algorithm if necessary. • Either SSTF or LOOK is a reasonable default algo

38. Summary • I/O Devices Types: • Many different speeds (0.1 bytes/sec to GBytes/sec) • Different Access Patterns: • Block Devices, Character Devices, Network Devices • Different Access Timing: • Blocking, Non-blocking, Asynchronous • I/O Controllers: Hardware that controls actual device • Processor Accesses through I/O instructions, load/store to special physical memory • Report their results through either interrupts or a status register that processor looks at occasionally (polling) • Device Driver: Device-specific code in kernel • Disks: • Latency Seek + Rotational + Transfer • Also, queuing time • Rotational latency: on average ½ rotation • Improve performance (decrease queuing time) via scheduling

39. Announcements • Homework 4 available later tonight • It is a programming assignment, so start early • Prelims graded • Mean 67.7 (Median 67), Stddev 14.2, High 96 out of 100! • Good job! • Re-grade policy • Submit written re-grade request to Nazrul. • Entire prelim will be re-graded. • We were generous the first time… • If still unhappy, submit another re-grade request. • Nazrul will re-grade herself • If still unhappy, submit a third re-grade request. • I will re-grade. Final grade is law.

40. Grade distribution

41. Question #2 • Algorithm • (1) Pick up a knife • (2) Pick a fork • (3) Cut out a slice of pizza and eat it • (4) Return the knife and fork to the pile • Correctness Constraints • wait for a knife and then a fork, in that order! • Key:Deadlock cannot occur since algorithm defines partial order thus, no circular waiting exists

42. Virtual index Offset 19 bits 13 bits PageTablePtrA frame # VDRWE Physical frame # Offset frame # VDRWE frame # VDRWE frame # VDRWE frame # VDRWE frame # VDRWE Question #3 13 and 19, respectively • 32 bit virtual address and 32-bit physical address, 8kB pages • #bits for offset? #bits for index? • Bytes required for PTE? Bytes required for page table? • 3 bytes and 219*3=1.5 MB, respectively 32 bits Virtual Address 13 bits 19 bits 0 1 Physical Address 2 3 32 bits … 219-1 19 bits 5 bits 24 bits = 3 bytes PTE

43. Virtual index Offset 19 bits 13 bits PageTablePtrA frame # VDRWE Physical frame # Offset frame # VDRWE frame # VDRWE frame # VDRWE frame # VDRWE frame # VDRWE Question #3 continued 13 and 19, respectively • 32 bit virtual address and 24-bit physical address, 8kB pages • #bits for offset? #bits for index? • Bytes required for PTE? Bytes required for page table? • 2 bytes and 219*2=1 MB, respectively 32 bits Virtual Address 13 bits 11 bits 0 1 Physical Address 2 3 24 bits … 219-1 (24-13=) 11 bits 5 bits 16 bits = 2 bytes PTE

44. Question #4 • Give a brief definition of the term “working set?” • Virtual memory pages touched within a window of time (or window of page references).

45. Question #5: CPU Scheduling • CPU Utilization w/ 10 I/O bound process and 1 CPU-bound • I/O bound compute for 1ms, sleep for 10ms • CPU bound computes indefinitely • Context-switch overhead is 0.1ms • CPU utilization w/ 1 ms quantum? • scheduler incurs a 0.1ms context-switching cost for every context-switch, regardless of process type • Cpu util = execTime/(execTime+contextSwitch) = 1/(1+0.1)=0.9090 • CPU utilization w/ 10 ms quantum? • #I/O*exI + #CPU*exC / (#I/O*(exI+cs) + #CPU*(exC+cs)) • 10*1 + 1*10 / (10*(1+0.1) + 1*(10+0.1)) • 20/(11+10.1) = 20/21.1 = 0.9478673

46. Question #5 continued • What strategy can a process employ to maximize the amount of CPU time allocated to that process? • Multilevel(-feedback) queue • Use a large fraction of assigned quantum • then relinquish the CPU before end of quantum • thus, increasing the priority associated with the process • Round robin • Use entire quantum • Or say no specific strategy • Alternatively, use more threads

47. How is the disk formatted? • After manufacturing disk has no information • Is stack of platters coated with magnetizable metal oxide • Before use, each platter receives low-level format • Format has series of concentric tracks • Each track contains some sectors • There is a short gap between sectors • Preamble allows h/w to recognize start of sector • Also contains cylinder and sector numbers • Data is usually 512 bytes • ECC field used to detect and recover from read errors

48. Cylinder Skew • Why cylinder skew? • How much skew? • Example, if • 10000 rpm • Drive rotates in 6 ms • Track has 300 sectors • New sector every 20 µs • If track seek time 800 µs • 40 sectors pass on seek Cylinder skew: 40 sectors

49. Formatting and Performance • If 10K rpm, 300 sectors of 512 bytes per track • 153,600 bytes every 6 ms  24.4 MB/sec transfer rate • If disk controller buffer can store only one sector • For 2 consecutive reads, 2nd sector flies past during memory transfer of 1st track • Idea: Use single/double interleaving

50. Disk Partitioning • Each partition is like a separate disk • Sector 0 is MBR • Contains boot code + partition table • Partition table has starting sector and size of each partition • High-level formatting • Done for each partition • Specifies boot block, free list, root directory, empty file system • What happens on boot? • BIOS loads MBR, boot program checks to see active partition • Reads boot sector from that partition that then loads OS kernel, etc.