I/O – Chapter 8 Introduction Disk Storage and Dependability – 8.2 Buses and other connectors – 8.4 I/O performance measures – 8.6

Input / Ouput devices __________________________________ –keyboard, mouse, printer, game controllers, … __________________________________ –hard drive, zip drive, … __________________________________ –music, video, …

Goals & Constraints

Taxonomy Behavior – –Input (____________) –Output (_______________________) –Storage (______________________________) Partner –Human or machine on other side? Data rate (speed) –______________________________________

Measures of Performance Response time –__________ – time a user must wait for task Bandwidth –_________________ per unit time

Anatomy of a Disk Drive

Vocabulary Head – the device that reads data from a disk Each disk is divided into ________ _______ called _________ Each track is made up of _________ sectors cylinder – volume of all _______ that lie under the heads at a given point on all surfaces nonvolatile – data that remains even when ______ is removed

Vocabulary seek – the act of positioning the _____ over the correct ________ rotational delay or latency – average latency to rotate the ______ over the correct _______ transfer time – time required to _________ a block of data disk controller – controls disk accesses

Example 1 - Performance What is the average time to read or write a 512-byte sector for a typical disk rotating at 10,000 RPM? The advertised average seek time is 6 ms, the transfer rate is 50 MB/sec, and the controller overhead is 0.2 ms. Assume that the disk is idle, so that there is no waiting time.

Reliability Reliability – measure of a continuously working system Availability – how often, on average, the system is working properly MTTF – Mean Time to Failure MTTR – Mean Time to Repair MTBF – Mean time between failures

Availability Availability – how often, on average, the system is working properly Availability = MTTF / (MTTF + MTTR)

Improving MTTF Fault avoidance Fault tolerance Fault forecasting

RAID - Redundant Arrays of Inexpensive Disks Shift from one large disk to several small disks Cheaper, smaller, faster Inherently less reliable Provide redundancy to counteract lower reliability

RAID 0 No redundancy!!! Only a performance increase Striping (interleaving) – allocation of logically sequential blocks to separate disks to increase performance Parallel access controlled by disk controller – computer knows nothing about it.

RAID 1 mirroring – write the identical data to multiple disks Requires twice as many disks as RAID 0 If a disk fails, use the backup copy, move to a working set of mirrored space.

RAID 3 Bit-interleaved parity Store only enough data to recover original Group N blocks Add one bit of parity – xor of all bits. Lost data can be reconstructed by looking at the rest of the bits in the group.

On a write Read all blocks of data in parity group Calculate new parity Write new block Write new parity

RAID 4 More efficient parity update On write: –Read old data –xor with new data –adjust parity –Write parity, Write new data

RAID 5 Rotate parity blocks around system Spread out writing (since parity always written)

Summary RAID 1 and RAID 5 most common 80% of server disks use RAID Repair: –hot swapping – replace disks with power on –Standby spares – spares included in system for immediate reconstruction of data

Connecting I/O Devices Much slower than processor / memory Support lots of heterogeneous devices

Bus control lines – send / receive commands data lines – transfer data processor-memory bus – fast, small bus connecting DRAM to processor I/O bus – slow, long bus connecting many devices to system through a controller.

Synchrony ___________________ – clock in control line, fixed protocol is relative to clock. _______________________ – no clock – must coordinate through hand-shaking to determine when data is ready to send / receive.

hand-shaking Series of steps used to coordinate bus transfers. Both parties must acknowledge they are ready before moving to next step. Control lines: ReadyReq: proc/device wants to read DataRdy: dev/proc is ready to send data Ack: acknowledge ReadReq or DataRdy

Metric Units Memory: GB = 2^30 I/O: GB = 10^9 Be careful when reading specs For this class, we will pretend that all use base-2 units. Throughput more important than latency Large database operations (TP – Transaction Processing)

Example 1 Execution time = 100 seconds 90 seconds CPU time, 10 sec I/O time CPU time improves by 50%/yr for 5 years I/O does not improve How much faster is program after 5 years? What percentage of new time is I/O?

Example 2 System A: –.005 sec per I/O op –4 overlapping I/O ops at a time System B: –.002 sec per I/O op –no overlapping I/O ops Which has the higher throughput?