codinggq , techniques for efficient, reliable networked...

Coding Techniques for Efficient, Reliable g q ,Networked Distributed Storage in Data Centers

Anwitaman DattaJoint work with Frédérique Oggier (SPMS)

School of Computer EngineeringNanyang Technological University

Infocomm Professional Development Forum 7th July 2011, Singaporey , g p

Self-* Aspects of Networked Distributed SystemsSelf Aspects of Networked Distributed Systems

Me, myself & SANDS& SANDS

htt //sands sce ntu d sg/

© 2011 A. DattaNTU Singapore

http://sands.sce.ntu.edu.sg/

http://www.ntu.edu.sg/home/anwitaman/

OutlineOo Preliminaries Data Center is at the center of the Cloud Data Center is at the center of the Cloud Failure is not an option, but …

• Known unknowns and unknown unknowns• Existing contingencies• Existing contingencies

o Fault-tolerance using erasure codes (ECs) Background

• RAID vs. ECs used as “Blackbox” o Erasure codes tailor-made for distributed networked

storageg Pyramid & hierarchical codes Regenerating codes Self-repairing codes


Self-repairing codes o Wrap-up

Preliminaries

o CloudPreliminaries- Cloud o Cloud

o Data center/Storage systemsDi t ib t d fil t

Cloud- Storage systems

Fault-tolerance

Tailor-made d Distributed file system

• Distributed data management– Key-Value store NoSQL Hadoop etc

codes

Wrap up

– Key-Value store, NoSQL, Hadoop, etc.

Reliable storage


What is the Cloud?Wo At least we can all agreePreliminaries

- Cloud o At least, we can all agree Cloud is something “big” and happening! It’s all of these

Cloud- Storage systems

Fault-tolerance

Tailor-made d Old It s all of these …

• … and some more!codes

Wrap up

SaaS

Old wine

IaaSData

center

PaaS %*$aaS


NIST Definition for Cloud ComputingN p gPreliminaries- Cloud o Cloud computing is a model for enablingCloud- Storage systems

Fault-tolerance

Tailor-made d

o Cloud computing is a model for enablingubiquitous, convenient, on-demand networkaccess to a shared pool of configurablecodes

Wrap up

access to a shared pool of configurablecomputing resources (e.g., networks,servers storage applications andservers, storage, applications, andservices) that can be rapidly provisionedand released with minimal managementand released with minimal managementeffort or service provider interaction.


Two Sides of the Cloud CoinPreliminaries- Cloud o Outside viewCloud- Storage systems

Fault-tolerance

Tailor-made d

o Outside view A “single/exclusive” entity

• Access through a “demilitarized zone”codes

Wrap up

• Access through a demilitarized zone …• API based• Agnostic to multi-tenancyg y

Infinite/elastic resources• Pay per use, on-demand, … y p

Browser based access (often)• Anytime, anywhere, any device …


Two Sides of the Cloud CoinPreliminaries- Cloud o Inside viewCloud- Storage systems

Fault-tolerance

Tailor-made d

o Inside view Pool of resources

• In flux: New compute units joining, old ones codes

Wrap up

p j gretiring

• Self-*: Load-balancing, fault-tolerance, auto-configuration,configuration, …

Multi-tenancy• Virtualization, transparent migration, …

Distributed file system and data-management• Google’s GFS, Amazon’s Dynamo, Yahoo!’s Pnuts


• Map Reduce/Hadoop, Pig, Chubby, …

The New Stack Preliminaries- Cloud

ApplicationsCloud

- Storage systems

Fault-tolerance

Tailor-made d

NoSQLe g Map‐Reduce

SQL Implementationse g PIG (relationalcodes

Wrap up

e.g., Map Reduce, Hadoop, BigTable, Hbase, Cassandra …

e.g., PIG (relational algebra), HIVE, …

Distributed File System (e.g., Key‐Value Store)

Distributed Physical Infrastructure: Storage/Compute Nodes

Reliable storage service


Distributed Physical Infrastructure: Storage/Compute Nodes

Disclaimer: This is a personal “view”, and may not be standard/universal

Data CenterPreliminaries- Cloud o Essentially a networked distributed storageCloud- Storage systems

Fault-tolerance

Tailor-made d

o Essentially a networked distributed storagesystem

codes

Wrap up


Source of topology: http://www.cisco.com/en/US/docs/solutions/Enterprise/Data_Center/vmware/VMware.html

Data Center Design EvolutionGen 1 DC Collocation

Gen 2 Gen 3 Gen 4 (future)Modular Data Center

g

Deployment Scale UnitDeployment Scale Unit

ServerServer RackRack ContainersContainers PrePre‐‐Assembled Assembled

Density and

ScalabilityTh d f

Right Time to Market,

ComponentsComponents


Capacityand Sustainability

Thousands of Servers

Lower TCO (PUE)Scalable Data Centers

Slide courtesy Roger Barga (Microsoft) from his P2P 2009 Keynote talk

Failure (of individual nodes) is inevitableFailure (of individual nodes) is inevitable

o But failure of the system is not an option!Preliminaries- Cloud o But, failure of the system is not an option!

o Solution: RedundancyCloud

- Storage systems

Fault-tolerance

Tailor-made dcodes

Wrap up


Is the Danger Real? Yesg

Preliminaries- CloudCloud- Storage systems

Fault-tolerance

Tailor-made dcodes

Wrap up

Cloud is


NSFW

Is the Danger Real? Yeso There is also the danger of data falling in

“wrong hands”, e.g. due to security breach

g

Preliminaries- Cloud g , g yCloud- Storage systems

Fault-tolerance

Tailor-made dcodes

Wrap up

o Security/privacy issues are out of the scope ofo Security/privacy issues are out of the scope of this talk@ SANDS we work on those issues also


A*Star TSRP project pCloud• http://sands.sce.ntu.edu.sg/pCloud/

Data Center Fault-TolerancePreliminaries o Faults are omnipresentFault-tolerance- Existing approaches- Has EC a role?

o Faults are omnipresent Hardware, network,

software, human, Tailor-made codes

Wrap up

misconfiguration, …o Cascade of failures in

interdependent networks Power failure => Network

switches stop workingswitches stop working Network failure => Control

system for power system i ff ti


ineffective

Redundancy Based Fault ToleranceRedundancy Based Fault Tolerance

o Replicate dataPreliminarieso Replicate data e.g., 3 or more copies In nodes on different racks

Fault-tolerance- Existing approaches- Has EC a role?

In nodes on different racks• Can deal with switch failures

o Power back up using battery between racks

Tailor-made codes

Wrap up

o Power back-up using battery between racks (Google)


Redundancy Based Fault ToleranceRedundancy Based Fault Tolerance

o Using “independent” physical infrastructurePreliminarieso Using independent physical infrastructure Over different availability zones (Amazon AZ)

• How independent are components in a complex


• How independent are components in a complex network?

Over multiple geographical regions

Tailor-made codes

Wrap up

p g g p g


Amazon’s AWS: Availability Zones yPreliminaries


Tailor-made codes

Wrap up

© 2011 A. DattaNTU Singapore Note: The recent (April 2011) AWS outage was the first region‐wide failure

Five Levels of RedundancyFive Levels of Redundancy

o PhysicalPreliminarieso Physicalo Virtual resource

A il bili


o Availability zoneo Region

Tailor-made codes

Wrap up

o Cloud


From: http://broadcast.oreilly.com/2011/04/the‐aws‐outage‐the‐clouds‐shining‐moment.html

At What Cost?At What Cost?Preliminaries

o Failure is not an option butFault-tolerance- Existing approaches- Has EC a role?

o Failure is not an option, but … … are the overheads acceptable?

Tailor-made codes

Wrap up


Reducing the Overheads of Redundancy

o Erasure codes

Reducing the Overheads of RedundancyPreliminaries

o Erasure codes Much lower storage overhead High level of fault tolerance


High level of fault-tolerance • In contrast to replication or RAID based systems

o Has the potential to significantly improve

Tailor-made codes

Wrap up

o Has the potential to significantly improve the “bottomline”

C it h t h th f d ? Can it however match the performance needs?• An open question


Erasure Codes (ECs)Erasure Codes (ECs)

o Originally designed for communication

B1

B

o Originally designed for communication EC(n,k)

Receive any k’ (≥ k) blocks

ge ata

O1

O2

B2 O1

O2

Encoding Decoding…… …

a = messag

nstruct D

a

Data

Reco

Ok Ok

Lost blocks


n encoded blocksOriginalk blocks k blocks

k

Bn

k

Erasure Codes for Networked StorageErasure Codes for Networked Storage

OB1 Retrieve any O

ject

O1

O2B2

…

k’ (≥ k) blocks

Data

O1

O2

Data = Obj

Encoding… …

construct

DecodingBl

D

Ok

… Lost blocks O i i l

Re

Ok

k blocks Bn

n encoded blocks

… Lost blocks Originalk blocks


(stored in storage devices in a network)

Static ResilienceStatic Resilienceo Replicated r times

F lt th t b t l t d 1

objectFor f=0.1its 10‐3Preliminaries

Faults that can be tolerated: r-1 Probability of failure: f r

Storage efficiency: 1/r

replic

replic

replic


g y Access: Find any one good replica

o Erasure coded (k of n)

ca ca ca

object

For f=0.13 of 9 code

Tailor-made codes

Wrap up

Faults that can be tolerated: n-k Probability of failure:

k k

object

Blk Blk Blk

its ~3*10‐6

Storage efficiency: k/n

jkjkn

k

jff

jknn

)1(

1

jkjkn

k

jff

jknn

)1(

1

Blk

Blk

Blk

Blk

Blk

Blk


Storage efficiency: k/n Access: Find k good blocks

o Assumption: Peer failure is i.i.d. with failure probability f

Replenishing Lost Redundancy for ECs p g yo Repairs needed for long-term resilience

B2

B1

…

Retrieve any k’ (≥ k) blocks

O1

O2Recreate lost blocks…

…Decoding EncodingBl

lost blocks

Re‐insert

Reinsert in (new) d i

Bn

… Lost blocks Originalk blocks

Ok storage devices, so that there is (again) n encoded blocks


k blocksn encoded blocks

o Repairs are expensive!

Can We Do Better?

o What is the best one can do (w.r.to repairs)?

Can We Do Better?Preliminaries

Minimize bandwidth usage per repair Minimize number of live nodes used per repair


o Erasure codes have some other drawbacks Coding/Decoding is “Expensive”

Tailor-made codes

Wrap upg g p

• In contrast to replication or RAID/XOR based systems• Systematic codes can help (with decoding/access)!

N t d t h l d b l i i l i !!– Not adequate when load-balancing is also an issue!!

More complex system design We do not attempt to address these explicitly


We do not attempt to address these explicitly• But, some solution we will arrive at will be amenable!

Can We Do Better?Can We Do Better?Preliminaries o Erasure codes tailor-made for distributed Fault-tolerance

Tailor-made codes- Pyramid codes

networked storage

- Regenerating Codes - Self-repairing codes

Wrap upPyramid codes

Wrap up

Regenerating codes

Self‐repairing codes


Pyramid & Hierarchical CodesPyramid & Hierarchical CodesPreliminaries o Essentially, “nested” use of erasure codesFault-tolerance


Oa1

…Oa2

Bla1 loca

redund

subgro


Wrap up

…Bl

an’

a2

Bg1

aldancy

gred

oup

Rep

global rethe c

Wrap up

…

Bgr’

1 globaldundancy

…

…

plicate cod

…

edundancy code group

Os1

Os2

localredundan…

Bls1

subgrou

…de group

from

ps


ncyBlsn’

up

Code group Multi-hierarchical extension

Pyramid & Hierarchical CodesPyramid & Hierarchical CodesPreliminaries

o Pros If `small’ number of faults

Fault-tolerance


• Communication restricted within the `hierarchy’ suffice• Progressively go higher-up for larger number of faults• Isolated faults can be repaired independently


Wrap up

p p y Naturally maps to hierarchical data-center design?

o ConsWrap up Asymmetry

• Different encoded blocks have different importance• Difficult to analyze• Complex algorithm (for decoding/repair) and system design


Another (essentially identical, but independent) proposal: Hierarchical Codes: How to Make Erasure Codes Attractive for Peer‐to‐Peer Storage Systems from Eurecom

Regenerating CodesRegenerating CodesPreliminaries o Network information flow based argumentsFault-tolerance


o Network information flow based arguments to determine “optimal” trade-off of storage/repair-bandwidth


Wrap up

sto age/ epa ba dw dt

Wrap up


Regenerating CodesRegenerating CodesPreliminaries o Example code (w/ Functional Repair)Fault-tolerance


p ( p ) Based on random linear network coding

• Exact repairing codes have also been proposed- Regenerating Codes - Self-repairing codes

Wrap upWrap up


Regenerating Codesg g

Preliminaries o ProsFault-tolerance


Network information flow analysis determines optimal (w.r.to repair bandwidth)

• Catch: Subject to MDS property of code (more on this, later)- Regenerating Codes - Self-repairing codes

Wrap up

Some proposed codes apply network coding on top of ECs

• Inherit the properties for EC for de/codingWrap up Inherit the properties for EC for de/coding

o Cons Codes for only specific points on trade-off curve

• Information flow analysis itself does not suggest any code Restrictive

• Some proposed codes can carry out repair only for one fault


p p y p y• Needs to contact all live nodes for repair to be optimal

Not simple: Algorithmic as well as system design

Self-repairing Codes (SRC)p g ( )

Preliminaries o Self-repairing codes are (n; k) codes s.t.Fault-tolerance


p g ( ) encoded fragments can be repaired directly

from other subsets of encoded fragments.- Regenerating Codes - Self-repairing codes

Wrap up

a fragment can be repaired from a fixed number of encoded fragments, independently of which

Wrap up specific blocks are missing• Analogous to erasure codes supporting

reconstruction using any n k losses independentlyreconstruction using any n - k losses, independently of which


Self-repairing Codes: Blackbox Viewp g

Preliminaries B1 Retrieve some Fault-tolerance


B2

…

k” (< k) blocks (e.g. k”=2)to recreate a lost block


Wrap up

Bl

Re‐insert

Wrap up

… Lost blocksReinsert in (new) storage devices, so h h ( )

Bn

n encoded blocks

… Lost blocks that there is (again) n encoded blocks


n encoded blocks(stored in storage

devices in a network)

Self-repairing Codesp g

Preliminaries o Two families of SRCs have been proposedFault-tolerance


p p


Wrap upWrap up


Self-repairing Codesp g

Preliminaries o There is at least one pair to repair a node, Fault-tolerance


p pfor up to (n -1)/2 simultaneous failures Parallel & fast repair of multiple fairs


Wrap up

p po PSRC (projective geometric SRC) example Data object split in four parts: PSRC(5 3)Wrap up Data object split in four parts: PSRC(5,3)


Toy Example: PSRC(5,3) repairy p ( , ) p

Preliminaries

Fault-tolerance

Tailor-made codes- Pyramid codes- Regenerating Codes - Self-repairing codes

Wrap up

(o1+o2+o4) + (o1) => o2+o4R i i t dWrap up (o3) + (o2+o3) => o2

(o1) + (o2) => o1+ o2

Repair using two nodes

Four pieces needed to regenerate two pieces

Say N1 and N3

(o +o +o ) + (o ) => o +o(o2) + (o4) => o2+ o4Repair using three nodes


(o1+o2+o4) + (o4) => o1+o2

Three pieces needed to regenerate two pieces

Say N2, N3 and N4

Toy Example: PSRC(5,3) reconstructiony p ( , )

Preliminaries

Fault-tolerance

Tailor-made codes- Pyramid codes- Regenerating Codes - Self-repairing codes

Wrap upWrap up o3o4(o3) + (o1+o3) => o1(o1) +(o4)+(o1+o2+o4) => o2(o1) +(o4)+(o1+o2+o4) > o2

Reconstruction, say using N3, N4 and N5


Symmetry in SRCsy y

o All encoded blocks have symmetric rolePreliminarieso All encoded blocks have symmetric role Equivalent importance of all blocks

• for both data reconstruction & repair

Fault-tolerance

Tailor-made codes- Pyramid codes • for both data reconstruction & repair

o Symmetry is goodE t l d t d d i l t


Wrap up Easy to analyze, understand and implement Simpler algorithm and system design

Wrap up


Maximum Distance Separable (MDS)?Maximum Distance Separable (MDS)?o SRC is not MDS (and can not be!)

D i ?Preliminaries

Does it matter?• Not much• In practice access will be “planned”

Fault-tolerance

Tailor-made codes- Pyramid codes • In practice, access will be “planned” …

PSRC needs less bandwidth than `optimal’ RGC!- Regenerating Codes - Self-repairing codes

Wrap up

This is with random access

Wrap up


PSRC(21,3)

Practical propertiesp po (Current) SRCs are not systematic PSRC is “like systematic”

Preliminaries PSRC is like systematic Need to contact more nodes (than k)

• To obtain systematic `pieces’

Fault-tolerance


• Same total bandwidth usage– Parallel download for access can even be an `advantage’

• `mixed’ strategies for access, i.e. get some systematic


Wrap up g , g ypieces, and some others …

– Power saving (by switching off nodes) strategies possible

Wrap up


Practical propertiesp po Self-repair implies

h “l ll d d bl ”Preliminaries

somewhat “locally decodable”• If access to only part of the whole object is desired

C di /d di i PSRC b h i

Fault-tolerance


o Coding/decoding in PSRC are both using XOR operations only


Wrap upWrap up


Outlook

o 2020: Self-repairing codes in a data-center near Preliminaries

you?o Ongoing:

Fault-tolerance

Tailor-made codes g g

Concepts/Implementation Prototype miniature

data center

Wrap up

data-center Template for

preassembled component of a modular 4G+ data center

o Interested to Follow:


http://sands.sce.ntu.edu.sg/CodingForNetworkedStorage/ Get involved: {anwitaman,frederique}@ntu.edu.sg

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