Remote Replication

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Remote ReplicationRemote Replication

Chapter 14

Section 3 : Business Continuity


Chapter Objective

After completing this chapter, you will be able to:

o Explain remote replication technologieso Synchronous and asynchronous

o Discuss host and array based remote replication o Functionalityo Differenceso Selecting the appropriate technology

o Discuss network options for remote replication


What is Remote Replication?

o Replica is created at remote siteo Addresses risk associated with regionally driven outages o Could be a few miles away or half way around the globe

o Modes of remote replication (based on RPO requirement)o Synchronous Replicationo Asynchronous Replication

Source site Remote site


Synchronous Replicationo A write must be committed to the source

and remote replica before it is acknowledged to the host

o Ensures source and remote replica have identical data at all times

o Write ordering is maintained o Replica receives writes in exactly the same

order as the source

o Synchronous replication provides the lowest RPO and RTO

o Goal is zero RPOo 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

o Response Time Extensiono Application response time will be

extendedo Data must be transmitted to target site

before write can be acknowledgedo Time to transmit will depend on distance

and bandwidth

o Bandwidtho To minimize impact on response time,

sufficient bandwidth must be provided at all times

o Rarely deployed beyond 200 km

Time

WritesMB/s

Required bandwidth

Typical workload

Max


Asynchronous Replication

o Write is committed to the source and immediately acknowledged to the host

o Data is buffered at the source and transmitted to the remote site later

o Some vendors maintain write orderingo Other vendors do not maintain write ordering, but

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

o Finite RPOo Replica will be behind the source by a finite

amounto Typically configurable

1

4

2

3

Data Write

Data Acknowledgement

Host

Target

Source


Asynchronous Replication: Bandwidth Requirement

o Response time unaffected

o Bandwidtho Need average bandwidth

o Bufferso Need sufficient buffers at the source to hold data before transmission to remote site

o Can be deployed over long distances

Average

Time

WritesMB/s

Required bandwidth

Typicalworkload


Remote Replication Technologies

o Host basedo Logical Volume Manager (LVM) based

o Support both synchronous and asynchronous modeo Log Shipping

o Storage Array basedo Support both synchronous and asynchronous modeo Disk Buffered - Consistent PITs

o Combination of Local and Remote Replication


LVM-basedo Duplicate Volume Groups at source and

target siteso All writes to the source Volume Group are

replicated to the target Volume Group by the LVM

o Can be synchronous or asynchronous mode

o In the event of a network failureo Writes are queued in the log file and sent to

target when the issue is resolvedo Size of the log file determines length of

outage that can be withstood

o Upon failure at source site, production can be transferred to target site

IP


LVM Based – Advantages and Limitation

o Advantageso The source and target sites do not need to use the same storage arrays

and RAID protection configurationo Response time issue can be eliminated with asynchronous mode (with the

drawback of extended RPO)o Specialized hardware / software is not required because LVM is available

in the OS running on the host

o Limitationso Extended network outages require large log fileso If log files fill up before outage is resolved, full synchronization is requiredo Replication process adds overhead on host CPUso Remote host must be continuously up and available


Host Based Log Shipping

o Offered by most database vendors

o Transactions to the source database are stored in logs

o Log files are transmitted to the remote host when the DBMS switch log fileso Happens at pre-configured time intervals or when a log file is full

o The remote host receives the logs and apply them to remote database

o Advantages:o Minimal overheado Low bandwidth requiremento Standby database is consistent up to the last applied log


Host Based Log Shipping

Original

Logs

Standby

Logs

IP


Source Array

Storage Array Based Remote Replication

o Replication performed by the array operating environmento Host CPU resources can be devoted to production operations instead of

replication operationso Arrays communicate with each other via dedicated channels

o ESCON, Fibre Channel or Gigabit Ethernet

o Replicas are on different arrayso Primarily used for DR purposeso 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

o No impact on response timeo Extended distances between arrayso 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

o Maintain write orderingo Some vendors attach a time stamp and sequence number with each write,

then send the writes to remote array o These writes are applied to the remote devices in exact order based on the

time stamp and sequence numbers

o Dependent write consistencyo Some vendors buffer the writes in the cache of the source array for a

period of time (between 5 and 30 seconds)o 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 buffero Closed buffer is then transmitted to the remote arrayo Remote replica will contain a consistent, re-startable image on the

application


Array Based – Disk Buffered Replication

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

o RPO usually in the order of hours

o Lower Bandwidth requirements

o Extended distance solution

Source Storage Array Target Storage Array

Local Replica Remote Replica

Local Replica

Source Host

Source Data


Remote Replicas – Tracking Changes

o Remote replicas can be used for BC operationso Typically remote replication operations will be suspended when the

remote replicas are used for BC operations

o During business operations changes will/could happen to both the source and remote replicaso Most remote replication technologies have the ability to track changes

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

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


Array Based – Which Technology?

o Synchronouso Is a must if zero RPO is requiredo Need sufficient bandwidth at all timeso Rarely above 125 miles

o Asynchronouso Extended distance solutions with minimal RPO (order of minutes)o No Response time elongationo Generally requires lower Bandwidth than synchronouso Must design with adequate cache/buffer capacity

o Disk buffered o Extended distance solution with RPO in the order of hourso Require lower bandwidth than synchronous or asynchronous


Three Site Replication

o Eliminates disadvantages of two site replication o Single site disaster leads to a window when there is no DR protection

o Data replicated to two remote sites

o Implemented in two wayso Three Site Cascade/Multi-hopo Three Site Triangle/Multi-target


Three Site Replication – Cascade/Multi-hop

o Synchronous + Disk Buffered

o 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

o Replicate from one storage array to any other storage array over the SAN/WANo Implement tiered storageo Data migrationo Remote vaulting

o Heterogeneous arrays support

o No impact to servers or the LAN

Hitachi

EMC SymmetrixEMC CLARiiON

HP

IBM

SAN/WAN


SAN Based Replication : Terminologieso Control Array: Array responsible for the replication operations

o Control Device: Device on controlling array to/from which data is being replicated

o Remote Array: Array to/from which data is being replicatedo Remote Device: Device on remote array to/from which data is being replicated

o Operationo Push: Data is pushed from control array to remote array o 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

o A dedicated or a shared network must be in place for remote replicationo Use ESCON (Enterprise System Connectivity) or FC (Fibre Channel) for

shorter distanceo For extended distances, an optical or IP network must be usedo Example of optical network: DWDM and SONET

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

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


Dense Wavelength Division Multiplexing (DWDM)

o 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

o 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)

o SONET is Time Division Multiplexing (TDM) technology

o Traffic from multiple sources of different speeds are organized into a frame and sent out onto SONET ring as an optical signal

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

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

SDH

STM-1 STM-16

SONET

OC3 OC48


Chapter Summary

Key points covered in this chapter:

o Modes of remote replicationo Synchronous and asynchronous mode

o Host based remote replicationo LVM based and log shipping

o Array based remote replicationo Synchronous, asynchronous and disk buffered o Three site replicationo SAN based remote replication

o Network options for remote replication

Additional Task

Research on EMC Remote Replication products


Check Your Knowledgeo What is the difference between Synchronous and

Asynchronous mode?

o Discuss one host based remote replication technology?

o Discuss one array based remote replication technology?

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

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


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Remote Replication

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