Remote Replication Chapter 14(9.3) ISMDR:BEIT:VIII:chap9.3:Madhu N:PIIT1.

ISMDR:BEIT:VIII:chap9.3:Madhu N:PIIT 1

Remote Replication

Chapter 14(9.3)

ISMDR:BEIT:VIII:chap9.3:Madhu N:PIIT - 2

Chapter Objective

After completing this chapter, you will be able to:• Explain remote replication technologies

– Synchronous and asynchronous• Discuss host and array based remote replication

– Functionality– Differences– Selecting the appropriate technology

• Discuss network options for remote replication


What is Remote Replication?• Replica is created at remote site

– Addresses risk associated with regionally driven outages – Could be a few miles away or half way around the globe

• Modes of remote replication (based on RPO requirement)– Synchronous Replication– Asynchronous Replication

Source site Remote site


Synchronous Replication• A write must be committed to the

source and remote replica before it is acknowledged to the host

• Ensures source and remote replica have identical data at all times– Write ordering is maintained

• Replica receives writes in exactly the same order as the source

• Synchronous replication provides the lowest RPO and RTO– Goal is zero RPO– RTO is as small as the time it takes to

start application on the target site

1

3

4

2

Data Write

Data Acknowledgement

Host

Target

Source


Synchronous Replication: Bandwidth Requirement

• Response Time Extension– Application response time will be

extended• Data must be transmitted to target

site before write can be acknowledged

• Time to transmit will depend on distance and bandwidth

• Bandwidth– To minimize impact on response

time, sufficient bandwidth must be provided at all times

• Rarely deployed beyond 200 km

Time

WritesMB/s

Required bandwidth

Typical workload

Max


Asynchronous Replication• Write is committed to the source

and immediately acknowledged to the host

• Data is buffered at the source and transmitted to the remote site later– Some vendors maintain write ordering– Other vendors do not maintain write ordering, but

ensure that the replica will always be a consistent re-startable image

• Finite RPO– Replica will be behind the source by a

finite amount– Typically configurable

1

4

2

3

Data Write

Data Acknowledgement

Host

Target

Source


Asynchronous Replication: Bandwidth Requirement

• Response time unaffected• Bandwidth

– Need average bandwidth• Buffers

– Need sufficient buffers• Can be deployed over long distances

Average

Time

WritesMB/s

Required bandwidth

Typicalworkload


Remote Replication Technologies

• Host based– Logical Volume Manager (LVM) based

• Support both synchronous and asynchronous mode

– Log Shipping• Storage Array based

– Support both synchronous and asynchronous mode

– Disk Buffered - Consistent PITs• Combination of Local and Remote Replication


LVM Based

• Duplicate Volume Groups at source and target sites– All writes to the source Volume Group

are replicated to the target Volume Group by the LVM

– Can be synchronous or asynchronous mode

• In the event of a network failure– Writes are queued in the log file and

sent to target when the issue is resolved– Size of the log file determines length of

outage that can be withstood • Upon failure at source site, production

can be transferred to target site

IP


LVM Based – Advantages and Limitations

• Advantages– Different storage arrays and RAID protection can

be used at the source and target sites– Response time issue can be eliminated with

asynchronous mode, with extended RPO• Limitations

– Extended network outages require large log files– CPU overhead on host


Host Based Log Shipping

• Offered by most database Vendors

• Advantages– Minimal CPU overhead– Low bandwidth requirement– Standby Database consistent

to last applied log

Original

Logs

Stand By

Logs

IP


Source Array

Storage Array Based Remote Replication• Replication performed by the array operating

environment– Host CPU resources can be devoted to production

operations instead of replication operations– Arrays communicate with each other via dedicated

channels• ESCON, Fibre Channel or Gigabit Ethernet

• Replicas are on different arrays– Primarily used for DR purposes– Can also be used for other business operations Target Array

DistanceSource Replica

DR ServerProduction

Server

IP/FC

Network


Array Based – Synchronous Replication

Network links

Write is received by the source array from host/server

Write is transmitted by source array to the target array

Target array sends acknowledgement to the source array

Source array signals write complete to host/server

Source Target


Array Based – Asynchronous Replication

• No impact on response time• Extended distances between arrays• Lower bandwidth as compared to Synchronous

Write is received by the source array from host/server

Write is transmitted by source array to the target array

Source array signals write complete to host/server

Target array sends acknowledgement to the source array

Network links

Source Target


Asynchronous Replication: Ensuring Consistency

• Maintain write ordering– Some vendors attach a time stamp and sequence number with each

write, then send the writes to remote array – Apply these writes to the remote devices in exact order based on the

time stamp and sequence numbers• Dependent write consistency

– Some vendors buffer the writes in the cache of the source array for a period of time (between 5 and 30 seconds)

– At the end of this time current buffer is closed in a consistent manner and the buffer is switched, new writes are received in the new buffer

– Closed buffer is then transmitted to the remote array– Remote replica will contain a consistent, re-startable image on the

application


Array Based – Disk Buffered Replication

• Local and Remote replication technologies can be combined to create consistent PIT copies of data on target arrays

• RPO usually in the order of hours• Lower Bandwidth requirements• Extended distance solution

Source Storage Array Target Storage Array

Local Replica Remote Replica

Local Replica

Source Host

Source Data


Remote Replicas – Tracking Changes

• Remote replicas can be used for BC operations– Typically remote replication operations will be suspended

when the remote replicas are used for BC operations• During business operations changes will/could

happen to both the source and remote replicas– Most remote replication technologies have the ability to

track changes made to the source and remote replicas to allow for incremental re-synchronization

– Resuming remote replication operations will require re-synchronization between the source and replica


Array Based – Which Technology?

• Synchronous– Is a must if zero RPO is required– Need sufficient bandwidth at all times– Rarely above 125 miles

• Asynchronous– Extended distance solutions with minimal RPO (order of minutes)– No Response time elongation– Generally requires lower Bandwidth than synchronous– Must design with adequate cache/buffer capacity

• Disk buffered – Extended distance solution with RPO in the order of hours– Require lower bandwidth than synchronous or asynchronous


Three Site Replication

• Eliminates disadvantages of two site replication – Single site disaster leads to a window when there

is no DR protection• Data replicated to two remote sites• Implemented in two ways

– Three Site Cascade/Multi-hop– Three Site Triangle/Multi-target


Three Site Replication – Cascade/Multi-hop• Synchronous + Disk Buffered

• Synchronous + AsynchronousBunker Site Remote Site


Local ReplicaSource Data

Synchronous

Remote Replica

Disk Buffered

Source Site

Bunker Site Remote Site


Local ReplicaSource Data

Synchronous

Remote Replica

Asynchronous

Source Site


Three Site Replication – Triangle/Multi-target

AsynchwithDifferentialResynchSOURCE

REMOTE

BUNKER

Sync

Async

SAN

SAN

SAN


SAN Based Remote Replication• Replicate from one storage

array to any other storage array over the SAN/WAN– Implement tiered storage– Data migration– Remote vaulting

• Heterogeneous arrays support

• No impact to servers or the LAN

Hitachi

EMC SymmetrixEMC CLARiiON

HP

IBM

SAN/WAN


SAN Based Replication: Terminologies

• Control Array: Array responsible for the replication operations– Control Device: Device on controlling array to/from which data is being

replicated• Remote Array: Array to/from which data is being replicated

– Remote Device: Device on remote array to/from which data is being replicated• Operation

– Push: Data is pushed from control array to remote array – Pull: Data is pulled to the control array from remote array

Control Array Remote Array

PUSH

PULLControl Device Remote Device

C CSAN/WAN


Network Options for Remote Replication

• A dedicated or a shared network must be in place for remote replication– Use ESCON or FC for shorter distance– For extended distances, an optical or IP network

must be used– Example of optical network: DWDM and SONET

• Protocol converters may require to connect ESCON or FC adapters from the arrays to these networks

– Native GigE adapters allows array to be connected directly to IP Networks


Dense Wavelength Division Multiplexing (DWDM)

• DWDM is a technology that puts data from different sources together on an optical fiber with each signal carried on its own separate light wavelength

• Up to 32 protected and 64 unprotected separate wavelengths of data can be multiplexed into a light stream transmitted on a single optical fiber

ESCON

Fibre Channel

Gigabit Ethernet

Optical Channels

Optical ElectricalOptical

Lambda λ


Synchronous Optical Network (SONET)• SONET is Time Division Multiplexing

(TDM) technology • Traffic from multiple subscribers is

multiplexed together and sent out onto SONET ring as an optical signal

• Synchronous Digital Hierarchy (SDH) similar to SONET but is the European standard

• SONET/SDH, offers the ability to service multiple locations, its reliability/availability, automatic protection switching, and restoration

SDH

STM-1 STM-16

SONET

OC3 OC48


Chapter Summary

Key points covered in this chapter:• Modes of remote replication

– Synchronous and asynchronous mode • Host based remote replication

– LVM based and log shipping• Array based remote replication

– Synchronous, asynchronous and disk buffered – Three site replication– SAN based remote replication

• Network options for remote replication

Remote Replication Chapter 14(9.3) ISMDR:BEIT:VIII:chap9.3:Madhu N:PIIT1.

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Transcript of Remote Replication Chapter 14(9.3) ISMDR:BEIT:VIII:chap9.3:Madhu N:PIIT1.