topics in computer system performance and reliability: storage systems!bianca/lec1.pdf · 2019. 1....

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CSC 2233:

Topics in Computer System Performance and Reliability: Storage Systems!

Note: some of the slides in today’s lecture are borrowed from a course taughtby Greg Ganger and Garth Gibson at Carnegie Mellon University

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Who am I?

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What makes storage systems so cool?

1. Combines so many topic areas:■ hardware meets OS meets networking meets distributed systems

meets security meets AI meets HCI…

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What makes storage systems so cool?

1. Combines so many topic areas2. This is where great jobs are!

■ Designers and implementers still needed● not just testing J

■ Continuing growth area for the future● The Internet is a network, but the web is a storage system● Strong existing companies: EMC, NetApp, …● Core competency for Internet services: Google, Microsoft, Amazon, …● and still support for start-ups

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What makes storage systems so cool?

1. Combines so many topic areas2. Great careers3. Still so much room to contribute:

■ performance actually matters here● in fact, it dominates other parts of system performance in many cases

■ … and reliability too■ storage management wide open■ and, storage starting to “take over” computation

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Amdahl’s Law

◆ Speedup limited to fraction improved■ obvious, but fundamental, observation

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50

90% reduction in BLUEyields only

45% reduction in total

◆ What does this mean for storage systems?

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Technology Trends

2000 2002 2004 2006 2008 2010Year

Nor

mal

ized

val

ue re

lativ

e to

200

0

1

10

100CPU Performance

Memory BandwidthDisk Bandwidth

Network Bandwidth

Network LatencyDisk Latency

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Consequence: storage performance dominates

• Assume 50 seconds CPU & 50 seconds I/O• CPU improves by 2X every 2 years

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Consequence: storage performance dominates

• Assume 50 seconds CPU & 50 seconds I/O• CPU improves by 2X every 2 years

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“I/O certainly has been lagging in the last decade”● Seymour Cray, 1976

“Also, I/O needs a lot of work”● David Kuck, 1988

“In 3 to 5 years, we will start seeing servers as peripherals to storage”

● SUN Chief Technology Officer, 1998

“Scalable I/O is perhaps the most overlooked area of high-performance computing R&D”

● Suggested R&D topic report for 2005-2009

Storage systems: fun quotes

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Remainder of the course

◆ Devices:■ Hard disk drives, solid state drives

◆ Local file systems■ File system organizations■ File system integrity/consistency

◆ NVM file systems◆ Distributed file systems◆ Parallel file systems◆ Extremely scalable storage (Google & Co)◆ Reliability & fault tolerance

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Logistics & Administratives

◆ Class time: Wed 10am – 12pm◆ Office hours:

■ By appointment◆ Class web page

■ www.cs.toronto.edu/~bianca/csc2233.html■ Still undergoing updates

◆ 11 weeks of lectures◆ Course project due end of the semester

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Grading

◆ 30% class participation■ Participation in class discussions

● (Read all papers prior to class)■ Class presentation of research paper■ Possibly quizzes (10%)

◆ 70% class project◆ No exams, no homework, no paper summaries

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Class project

◆ Can be done in team of two or alone■ Start looking for a partner now!

◆ We will suggest possible projects (see course web page)◆ Output: workshop quality research paper (10-12 pages)

■ Even better: conference quality paper■ Use latex template on course web page■ All reports will be published as tech-report

◆ We will help you get there --- multiple milestones:◆ And meetings with TA/instructor

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Paper presentation

◆ Each of you will present at least one paper in class◆ Format of the presentation:

■ 25 min presentation of paper contents■ 5-15 min paper review

● Good points● Bad points

■ 10 min class discussion that you lead!● Prepare questions!

◆ Keep the class engaged!

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Paper presentation

◆ What I do not want:■ A long laundry list of all things the paper did

◆ What I do want:■ A lecture style presentation of the paper

● Including background material your fellow class mates might need to understand the paper

■ A critical discussion of the paper● Strength & Weaknesses● Prepare questions!

◆ What you get:■ Feedback!

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Purpose of presentation

◆ Wrong answers:■ “To give a verbal version of the paper, cramming all its content into

30 min”■ “To impress people with your technical depth and thoroughness”

◆ In fact, no one cares about these things■ The goal is to filter out the main points of the paper and present

them well■ By the end, everybody in the audience should remember 2-3 take-

home messages

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What’s on each slide?

◆ Control level of detail

◆ Each slide should have one basic point◆ There should NOT be tons of text◆ Use sentence fragments◆ Use pictures everywhere you possibly can!

■ A picture says more than 1000 words■ Saves text and thus slides■ Much easier to process

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Rest of today: Some review …

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What are storage systems all about?

◆ Memory/storage hierarchy

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Memory/storage hierarchies

◆ Balancing performance with cost■ Small memories are fast but expensive■ Large memories are slow but cheap

◆ Exploit locality to get the best of both worlds■ locality = re-use/nearness of accesses■ allows most accesses to use small, fast memory

Capacity

Performance

L1/2CACHE

L3CACHE

DRAM

SSD

HARD DISK

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Example memory hierarchy values

Notice the huge access time gap

between DRAM and disk

SSDs(tens of microsecs)

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What are storage systems all about?

◆ Memory/storage hierarchy■ Combining many technologies to balance costs/benefits■ For long time not the focal point of storage system design

● More interesting in recent years with SSDs and NVMs arriving on the market

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What are storage systems all about?

◆ Memory/storage hierarchy■ Combining many technologies to balance costs/benefits■ For long time not the focal point of storage system design

● More interesting in recent years with SSDs and NVMs arriving on the market

◆ Persistence■ Storing data for lengthy periods of time■ To be useful, it must also be possible to find it again later

● this brings in data organization, consistency, and management issues■ This is where the serious action is

● and it does relate to the memory/storage hierarchy

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Why persistence is important

◆ Some statistics:■ Among companies who lose data in a disaster, 50% never re-open

and 90% are out of business within two years■ Even smaller incidents can be costly

● Reproducing some tens of megabytes of accounting data can take several weeks and cost tens of thousands of dollars

■ Bad PR!

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Storage System

Application

Bob1Bob2Bob3Bob4

Bob1

Bob2Bob3Bob4

Bob2

Bob3Bob4

Bob3

Bob4

Bob4

Application gives data objects & their

IDs to storage

What is a storage system: Big Picture

The storage systemkeeps the data objectsand returns one upon

request (by ID)Bob2

Bob1

Bob3Bob4

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Storage Systems & Interfaces

◆ What is a “Storage System”?■ Hardware (devices, controllers, interconnect) and Software (file

system, device drivers, firmware) dedicated to providing management of and access to persistent storage.

◆ One view: defined by collection of interfaces

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Program PhysicalMedia

Filesystem

Devicedriver

I/O controller

High level of abstraction No abstraction

Storage Software Interfaces

Understands files and directories

HDD understands platters, cylinders, tracks, sectors

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What’s inside a disk?

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Disk structure – top view of single platter

◆ Surface organized into tracks◆ Tracks organized into sectors

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Disk service time components

◆ Components:■ Seek■ Rotational latency■ Data transfer

After BLUE read Seek for RED Rotational latency After RED read

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Seek time

◆ Time required to move head over desired track◆ A real seek profile:

◆ Note that this is not linear!

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Seek time

◆ Seek times not linear because they have up to four components:■ Accelerate■ Coast at max velocity

● If going far enough to reach max velocity■ Decelerate■ Settle onto correct trace

● Takes extra time to settle before writing

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What is the average seek time?

◆ Watch out for misrepresentations

◆ What it is not:■ Seek time for average of possible distances■ Seek time for any LBN to any other

◆ What it is:■ Depends on workload■ Very different for sequential versus random workloads

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Where does the disk head’s time go?

◆ Seek time, rotational latency, transfer time?

Random 4KB requests

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Impact of request sizes?

◆ Seek time, rotational latency, transfer time?

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Impact of locality?

◆ Seek time, rotational latency, transfer time?

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Where does the disk head’s time go?

◆ Seek time: 1– 6ms, depending on distance■ Improving at 7-10% per year

◆ Rotation speeds: 7,200-15,000 RPM■ Average latency of 2-4ms■ Improving at 7-10% per year

◆ Data rates: 60-100 MB/s■ Average sector transfer time of 25us■ Improving at 30-40% per year

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What’s inside a disk?

◆ The mechanics:

◆ The electronics (just like a small computer):■ A processor■ DRAM ■ Control ASIC

What are all those needed for?

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Disk drive – what’s in a sector?

◆ Data■ Typically 512 bytes

◆ Sync bytes = pattern to notify controller that data follows◆ Header (ID information)

■ Cylinder, head, and sector number◆ ECC (error correcting codes)

■ At such high densities, problems occur■ ECC detects and corrects on the fly■ “Tri-state guarantee” of sector writes

● All written● All not written● Sector destroyed● NEVER: partially modified

● Servo = bit pattern used for centering on track

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How is functionality implemented?

◆ Some in ASIC logic:■ Error detection and correction■ Servo processing■ Motor-seek control

◆ Some in firmware running on control processor

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How is functionality implemented?

◆ Some in ASIC logic:■ Error detection and correction■ Servo processing■ Motor-seek control

◆ Some in firmware running on control processor■ Request processing, queueing, scheduling■ LBN to PBN mapping

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How to map LBN to PBN

65 7 12 23……◆ The view of the OS

◆ The reality:

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LBN to physical mapping for single surface

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Extending the mapping to a multi-surface disk

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First complication: zones

Real disks don’t have constant number of sectors per track

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Multiple “zones”

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Computing physical location from LBN

……. …….

An example zone breakdown

◆ First, figure out which zone contains the LBN■ i.e. which cylinder

◆ Then determine surface number◆ Then determine sector number

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Second complication: defect management

◆ Disks keep spare sectors◆ Those are used in case portions of the media become

unusable (both before and after installation)

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Second complication: defect management

◆ First approach: remap broken sector, don’t touch anything else

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Second complication: defect management

◆ Second approach, “slip” mapping past broken sector

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Third complication: skew

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Third complication: skew

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Third complication: skew

◆ It takes time to switch from one track to another■ Sequential transfers suffer full rotation

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Same request with track skew of one sector

◆ Track skew prevents unnecessary rotation

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Same request with track skew of one sector

◆ Track skew prevents unnecessary rotation

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How is functionality implemented?

◆ Some in ASIC logic:■ Error detection and correction■ Servo processing■ Motor-seek control

◆ Some in firmware running on control processor■ Request processing, queueing, scheduling■ LBN to PBN mapping

● Zones● Defects● Skew