© 2009 EMC Corporation. All rights reserved. Remote Replication Module 3.4.

© 2009 EMC Corporation. All rights reserved.

Remote ReplicationRemote Replication

Module 3.4

© 2009 EMC Corporation. All rights reserved. Remote Replication - 2

Remote Replication

After completing this module, 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 Limitation

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

Asynch

with

Differential

ResynchSOURCE

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

Application and OS independent

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


Module Summary

Key points covered in this module:

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


Concept in Practice – EMC Remote Replication

EMC Symmetrix Arrays– EMC SRDF/Synchronous

– EMC SRDF/Asynchronous

– EMC SRDF/Automated Replication

EMC CLARiiON Arrays– EMC MirrorView/Synchronous

– EMC MirrorView/Asynchronous

EMC SAN Copy– SAN based remote replication solution for EMC CLARiiON


Check Your Knowledge

What is the difference between Synchronous and Asynchronous mode?

Discuss one host based remote replication technology?

Discuss one array based remote replication technology?

What are differences in the bandwidth requirements between the array remote replication technologies discussed in this module?

Discuss the effects of a bunker failure in a three-site replication for the following implementation:− Multihop—synchronous + disk buffered− Multihop—synchronous + asynchronous− Multitarget

© 2009 EMC Corporation. All rights reserved. Remote Replication Module 3.4.

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