November 4, 2013 Sam Siewert

CS A320 Operating Systems for Engineers

Lecture 10 – Introduction to Drivers and I/O to Supplement MOS Chapter 5

Introduction to Drivers

For Linux and in General

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Hardware View of Device Interfaces Analog I/O – DAC analog output: servos, motors, heaters, ... – ADC analog input: photodiodes, thermistors, ...

Digital I/O – Direct TTL I/O or GPIO – Digital Serial (I2C, SPI, ... - Chip-to-Chip) – Bus Interfaces

Parallel – PCI 2.x, PCI-X, SCSI, etc (32-bit, 64-bit, synchronous parallel transfer)

Differential Serial – USB – Infiniband – gigE / 10GE Ethernet – Fiber Channel – SAS/SATA

Analog NTSC Signal for Camera

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As Seen on Agilent MSO – Composite Luminance and Chorma Signals at 6MhZ – Interlaced ODD/EVEN line raster

Example NTSC Digital Output

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As Seen on Texas Instruments Decoder Digital Output for YCrCb by Intronix Logic Analyzer – Clock, Data, Control at Logic Levels – 6 MhZ NTSC decode, Sampled at 10MhZ or Higher

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Software View of Drivers Character – Register Control/Config, Status, Data – Typical of Low-Rate I/O Interfaces (RS232) – Linux User Space Buffer Drivers (Direct IO) – e.g.

SCSI Generic Block – FIFOs, Dual-Port RAM and DMA – Typical of High-Rate I/O Interfaces (Network, Storage) – Only Interface for 512 Byte LBA/Sector HDDs

Network – Driver Stacks – OSI 7 Layer Model (Phy, Link, Network, Transport, Session,

Presentation, Application) – TCP/IP/Ethernet/Cat-6e

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Linux Char Driver Design Application Interface – Application Policy – Blocking/Non-Blocking – Multi-thread access – Abstraction

Device Interface – SW/HW Interface – Immediate Buffering – Interrupt Service

Routine App/Device Interface Hardware Device

Application(s)

ISR

SemGive Input Ring-Buffer

Output Ring-Buffer

If Output Ring-Buffer Full then

{SemTake or EAGAIN}

else {Process and Return}

If Input Ring-Buffer Empty then

{SemTake or EAGAIN}

else {Processand Return}

open/close, read/write, creat, ioctl EAGAIN, Block,

Data, Status

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Cached Memory and DMA

Cache Coherency – Making sure that cached data and memory are in sync – Can become out of sync due to DMAs and Multi-Processor

Caches – Push Caches Allow for DMA into and out of Cache Directly – Cache Snooping by HW may Obviate Need for Invalidate

Drivers Must Ensure Cache Coherency – Invalidate Memory Locations on DMA Read Completion – Flush Cache Prior to DMA Write Initiation

IO Data Cache Line Alignment – Ensure that IO Data is Aligned on Cache Line Boundaries – Other Data That Shares Cache Line with IO Data Could

Otherwise Be Errantly Invalidated

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Advantages of Abstracted Driver

Portability – If SW/HW Interface changes, change BH – If Application Interface changes, change TH

Testability (Test BH and TH Separately) Single Point of Access and Maintenance Enforce Multi-Thread Usage Policies Separate Buffering and ISR from Usage Common Application Entry Points Scheduled I/O (Most Work in Task Context rather than ISR Context)

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Linux Driver Writer Resources “Linux Device Drivers – 3rd Ed.”, by J. Corbet, A. Rubini, G. Kroah-Hartman, 2005, (0-596-00590-3), O’Reilly, publisher link, E-book link "PCI System Architecture", Tom Shanley and Don Anderson, 4th Edition, 1999, (ISBN 0-201-30974-2) MindShare, Inc., E-book link, publisher link, retailer link, library link.

Digital Media Filesystems Three Types of Media Storage – Direct Attached Storage – e.g. SATA (Serial ATA) – Network Attached Storage – e.g. NFS – Storage Area Networks – e.g. SAS (Serial Attached SCSI), Fiber

Channel

Flash / RAM based SSD Still 10x++ More Costly than Spinning Media – Predictions for Demise of HDDs and RAID? – Cost is the Driver

Fast Storage is Either SSD, RAID or Hybrid

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RAID Operates on LBAs/Sectors (Sometimes Files)

SAN/DAS RAID NAS – Filesystem on top of RAID RAID-10, RAID-50, RAID-60 – Stripe Over Mirror Sets – Stripe Over RAID-5 XOR Parity Sets – Stripe Over RAID-6 Reed-Soloman or Double-Parity Encoded Sets

EVEN/ODD Row Diagonal Parity Minimum Density Codes (Liberation) Reed-Solomon Codes

– Generalized Erasure Codes Cauchy Reed-Solomon, LDPC (Low Density Parity Codes), Weaver/Hover MDS (Maximal Distance Seperation) – For each Parity Device, Another Level of Fault Tolerance is Provided

– Larger Drives (Multi-terabyte), Larger arrays (100’s of drives), and Cost Reduction are Driving RAID6 and Higher Levels

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RAID-10

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A1 A1 A2 A2 A3 A3 A4 A4 A5 A5 A6 A6

RAID-1 Mirror RAID-1 Mirror RAID-1 Mirror

RAID-0 Striping Over RAID-1 Mirrors

A7 A7 A8 A8 A9 A9 A10 A10 A11 A11 A12 A12

A1,A2,A3, … A12

RAID5,6 XOR Parity Encoding

MDS Encoding, Can Achieve High Storage Efficiency with N+1: N/(N+1) and N+2: N/(N+2)

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0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

100.0%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Stor

age

Effic

ienc

y

Number of Data Devices for 1 XOR or 2 P,Q Encoded Devices

RAID6

RAID5

RAID-50

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A1

RAID-5 Set RAID-5 Set

B1 C1 D1 P(ABCD)

E1 F1 G1 H1 P(EFGH)

I1 J1 P(IJKL) K1 L1 M1 P(MNOP) N1 P1 O1

P(QRST) Q1 R1 S1 T1

A2 B2 C2 D2 P(ABCD)

E2 F2 G2 H2 P(EFGH)

I2 J2 P(IJKL) K2 L2 M2 P(MNOP) N2 P2 O2

P(QRST) Q2 R2 S2 T2

RAID-0 Striping Over RAID-5 Sets

A1,B1,C1,D1,A2,B2,C2,D2,E1,F1,G1,H1,…, Q2,R2,S2,T2

A1

RAID-6 Set RAID-6 Set

B1 C1 D1 P(ABCD)

E1 F1 G1 P(EFGH)

I1 J1 P(IJKL) K1 M1 P(MNOP) N1 O1 P(QRST) Q1 R1 S1

RAID-0 Striping Over RAID-6 Sets

A1,B1,C1,D1,A2,B2,C2,D2,E1,F1,G1,H1,…, Q2,R2,S2,T2

Disk5 Disk1 Disk2 Disk3 Disk4

Q(EFGH)

Disk6

H1 QABCD)

Q(IJKL)

Q(MNOP)

Q(QRST)

L1 P1 T1

A2 B2 C2 D2 P(ABCD)

E2 F2 G2 P(EFGH)

I2 J2 P(IJKL) K2 M2 P(MNOP) N2 O2 P(QRST) Q2 R2 S2

Disk5 Disk1 Disk2 Disk3 Disk4

Q(EFGH)

Disk6

H2 QABCD)

Q(IJKL)

Q(MNOP)

Q(QRST)

L2 P2 T2

RAID-60 (Reed-Solomon Encoding)

How RAID Relates to Erasure Codes

Erasure Codes Applied to Disk or SSD Devices

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RAID is an Erasure Code

RAID-1 is an MDS EC (James Plank, U. of Tennessee)

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Comparison of ECs

Data Devices = n Coding Devices = m Total = m+n Storage Efficiency: R=n/(n+m)

– RAID1 2-Way, R=1/(1+1)=50%, MDS=1, Reads 2x Speed-up, 1x Write – RAID1 3-Way, R=1/(1+2)=33%, MDS=2, 3x Read, 1x Write – RAID10 with 10 sets, R=10/(10+10)=50%, MDS=1, 20x Read, 10x Write – RAID5 with 3+1 set, R=3/(3+1)=75%, MDS=1, 3x Read (Parity Check?), RMW Penalty,

Striding Issues – RAID6 with 7+2 set, R=5/(5+2)=71%, MDS=2, 5x Read, Reed-Solomon Encode on Write

and RMW Penalty – Beyond RAID6?

Cauchy Reed-Solomon Scales, but Encode, Decode Complexity High Low Density Parity Codes, Simpler, but not MDS

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Read, Modify Write Penalty

Any Update that is Less than the Full RAID5 or RAID6 Set, Requires 1. Read Old Data and Parity – 2 Reads 2. Compute New Parity (From Old & New Data) 3. Write New Parity and New Data – 2 Writes Only Way to Remove Penalty is a Write-Back Cache to Coalesce Updates and Perform Full-Set Writes Always

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A1

RAID-5 Set

B1 C1 D1 P(ABCD)

E1 F1 G1 H1 P(EFGH)

I1 J1 P(IJKL) K1 L1 M1 P(MNOP) N1 P1 O1

P(QRST) Q1 R1 S1 T1

Write A1 P(ABCD)new=A1new xor A1 xor P(ABCD) A1 B1 C1 D1 P(ABCD) 0 0 0 0 0 0 0 0 1 1 0 0 1 0 1 0 0 1 1 0 0 1 0 0 1 0 1 0 1 0 0 1 1 0 0 …

Conclusion Deeper Dive Into Erasure Codes (James Plank FAST Presentation) Lab 3 Discussion Linux RAID Demos Driver Discussion

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