1. IO and RAID

2. Input/Output 5.1 Principles of I/O hardware
5.2 Principles of I/O software
5.3 I/O software layers
5.4 Disks
5.5 Clocks
5.6 Character-oriented terminals
5.7 Graphical user interfaces
5.8 Network terminals
5.9 Power management

3. Device Controllers I/O devices have components:
mechanical component
electronic component
The electronic component is the device controller
may be able to handle multiple devices
Controller's tasks
convert serial bit stream to block of bytes
perform error correction as necessary
make available to main memory

4. Memory-Mapped I/O: merits
all code can be written in C
-test&set on control register
data buffer in memory
control registers are on ports
IN REG, PORT
OUT PORT, REG

5. Memory-Mapped I/O: demerits
cached IO registers can be disastrous
complex bus architectures
case (b) is more difficult for MM IO

6. Direct Memory Access (DMA)
physical or virtual address?

7. Goals of I/O Software (1)
Device independence
programs can access any I/O device
without specifying device in advance
(floppy, hard drive, or CD-ROM)
Uniform naming
name of a file or device a string or an integer
not depending on which machine
Error handling
handle as close to the hardware as possible
synchronous vs asynchronous
buffered or not

8. Goals of I/O Software (2)
Synchronous vs. asynchronous transfers
blocked transfers vs. interrupt-driven
Buffering
data coming off a device cannot be stored in final destination
Sharable vs. dedicated devices
disks are sharable
tape drives would not be

9. Programmed I/O
copy_from_user(buffer, p, count) /* p is the kernel buffer */
for(i=0; i<count; i++) {
while(*printer_status_register != READY); /* polling */
*printer_data_register = p[i]; }

10. Interrupt-Driven I/O syscall ends up with calling the scheduler (a)
io request
interrupt
printer
syscall ends up with calling the scheduler (a)
the device raises an interrupt when it is ready
the interrupt is handled like (b)

11. I/O Using DMA
syscall (a) and interrupt handler (b)

12. Layers of the I/O Software System
I/O Software Layers
Layers of the I/O Software System

13. Interrupt Handlers (1)
Save regs not already saved by interrupt hardware
Set up context(MMU) for interrupt service procedure
Set up stack for interrupt service procedure
Ack interrupt controller, reenable interrupts
Copy registers from where saved
Run service procedure
Set up MMU context for process to run next
Load new process' registers
Start running the new process
top half
bottom half
interrupt handler

14. Device Drivers
device driver

15. Device Drivers in User Space
Advantages
all user level utilities can be used
libraries, debugger
an error affects only the driver, not the whole kernel
easy installation (dynamic and static)
Disadvantages
interrupts are not available
direct memory is not possible
slow (user/kernel switching)
usually not reentrant

16. Device-Independent I/O Software (1)
Uniform interfacing for device drivers
Buffering
Error reporting
Allocating and releasing dedicate devices
Providing a device-independent block size
io software
Functions of the device-independent I/O software

17. Device-Independent I/O Software (2)
Standard interface eases development of device driver
naming like a file eases protection

18. Input Buffering (a) Unbuffered input: process one char at a time
(b) Buffering in user space: buffer may be paged out
(c) Buffering in the kernel followed by copying to user space: buffer may not has been copied yet
(d) Double buffering in the kernel

19. Copy Operations Due to Buffering
Networking may involve many copies

20. Review
Layers of the I/O system and the main functions of each layer

21. Disk Hardware (1) model Baracuda ATA II cheetah 73 capacity 30GB 73GB
plates #
heads #
RPM
sector size 512B same
sector/track
track/in
seek time
read ms 5.85ms
write 9.5ms 6.35ms
track to track(r) ms 0.6ms
track to track(w) 1.9ms 0.9ms

22. Disk Hardware (2) Physical geometry of a disk with two zones
A possible virtual geometry for this disk

23. RAID no redundancy same as SLED for a single sector request mirroring
spread byte or word
disks should be synchronized
Hamming code

24. Disk Hardware (4) same as RAID-2 parity instead of Hamming
deals with single failure

25. Disk Formatting
cylinder skew hides track-to-track seek latency

26. Disk Interleaving no interleaving accessing consecutive sectors
1. move a sector to buffer
2. copy buffer to memory
3. move next sector….
cannot utilized contiguous allocation
single/double interleaving
better buffer entire track instead of a sector

27. Disk Arm Scheduling Algorithms (1)
Time required to read or write a disk block determined by 3 factors
Seek time
Rotational delay
Actual transfer time
Seek time dominates
Error checking is done by controllers

28. Error Handling A disk track with a bad sector
Substituting a spare for the bad sector
Shifting all the sectors to bypass the bad one

29. Stable Storage stable storage write to mirrored disks
writes correct value or leaves the disk intact
write to mirrored disks
write to disk 1 and verify
do the same to disk 2
non-volatile RAM speeds up this operation

30. RS-232 Terminal Hardware
An RS-232 terminal communicates with computer 1 bit at a time
Called a serial line – bits go out in series, 1 bit at a time
names: /dev/ttyx, COM1 and COM2 ports
Computer and terminal are completely independent

31. Input Software (1) Central buffer pool
Dedicated buffer for each terminal

32. Display Hardware (1) Memory-mapped displays
Parallel port
Memory-mapped displays
driver writes directly into display's video RAM

33. Input Software Keyboard driver delivers a number
driver converts to characters
uses a ASCII table
Exceptions, adaptations needed for other languages
many OS provide for loadable keymaps or code pages

34. Clients and servers in the M.I.T. X Window System

35. The SLIM Network Terminal (1)
The architecture of the SLIM terminal system

36. The SLIM Network Terminal (2)
Messages used in the SLIM protocol from the server to the terminals

37. Power consumption of various parts of a laptop computer
Power Management (1)
Power consumption of various parts of a laptop computer

38. Power management (2)
The use of zones for backlighting the display

39. Power Management (3) Running at full clock speed
Cutting voltage by two
cuts clock speed by two,
cuts power by four

40. Power Management (4) Telling the programs to use less energy Examples
may mean poorer user experience
Examples
change from color output to black and white
speech recognition reduces vocabulary
less resolution or detail in an image

41. AutoRAID Problems with RAID difficult to configure
requires detailed knowledge of disks and workload
too many parameters to set
difficult to change the configuration
difficult to add disks
RAID 5 preserve an extra disk to cope with a disk failure (hot spare)
we don’t know if this disk works since it is not used in normal operation

42. AutoRAID The solutions Offer a hierarchy of levels
store active data in mirrored disks and store inactive data in level 5 arrays
depending the activity, data are moved across this hierarchy
the active set must be
small to be fitted into the mirrored disk
be unchanged for a long time to prevent frequent data movement

43. Hierarchy Implementation
manually by the system administrator
error-prone
cannot adapt to rapidly changing access patterns
requires highly skilled person
difficult to add new disks
file system
best place since it is aware of access patterns for each file and it is a software
not in customer’s hand
array controller (behind scsi controller)
devoid of the knowledge of access patterns
make the array look like a normal disk

44. AutoRAID Features
Mapping: maps host block address to physical location
Mirroring and RAID 5
use mirroring until run short of disk space,
if space is needed, store some data as RAID 5
keep data in RAID 1 be at least 10% of all data
transfer data by the frequency of accesses
automatic and done in idle time
online disk upgrade
dual controllers
active use of hot spares
when a disk fails, some data in the level 1 are moved to level 5 to make a room for reconstruction

45. Hardware Configuration

46. Data Layout
LUN = a number of PEX
RB is visible to clients

47. Data Layout(2)

48. Mapping Tables

49. Migration from level 5 to level 1 (from inactive to active)
when RB is written
from RAID 1 to RAID 5
in background
if the data has not been changed for a long time
if space is needed in Mirror

50. Review shows a perfect research and development
hardware development in parallel with the simulator
an accurate simulator makes it possible to calibrate the design decision before they committed to making a product
Comparison with CLARiiON and JBOD(Just Bunch of Disks)
Microbenchmark results do not give much intuition
Would be nice to show what happens when AutoRAID degrades to the mirrored mode
Not much why, mostly what
Does not really tell us much about the statistical significance