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Lecture 30. Multiple Replicas. Remote Replication. DR in practice. Chapter Objectives. 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

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Multiple replicas remote replication dr in practice

Lecture 30

Multiple Replicas. Remote Replication. DR in practice.


Chapter objectives
Chapter Objectives

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

Remote Replication


What is 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

Remote site

Source site

Remote Replication


Synchronous replication

1

4

2

3

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

Source

Host

Data Write

Data Acknowledgement

Target

Remote Replication


Synchronous replication bandwidth requirement

Required bandwidth

Typical workload

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

Max

Writes

MB/s

Time

Remote Replication


Asynchronous replication

1

2

3

4

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

Source

Host

Data Write

Data Acknowledgement

Target

Remote Replication


Asynchronous replication bandwidth requirement

Typical

workload

Asynchronous Replication: Bandwidth Requirement

  • Response time unaffected

  • Bandwidth

    • Need average bandwidth

  • Buffers

    • Need sufficient buffers

  • Can be deployed over long distances

Required

bandwidth

Writes

MB/s

Average

Time

Remote Replication


Remote replication technologies
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

Remote Replication


Lvm based
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

Remote Replication


Lvm based advantages and limitations
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

Remote Replication


Host based log shipping
Host Based Log Shipping

Logs

  • Offered by most database Vendors

  • Advantages

    • Minimal CPU overhead

    • Low bandwidth requirement

    • Standby Database consistent

      to last applied log

IP

Original

Logs

Stand By

Remote Replication


Storage array based remote replication
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

Source Array

Target Array

IP/FC

Network

Distance

Source

Replica

Production Server

DR Server

Remote Replication


Array based synchronous replication

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

Array Based – Synchronous Replication

Network links

Source

Target

Remote Replication


Array based asynchronous replication

Write is received by the source array from host/server

Source array signals write complete to host/server

Target array sends acknowledgement to the source array

Write is transmitted by source array to the target array

Array Based – Asynchronous Replication

  • No impact on response time

  • Extended distances between arrays

  • Lower bandwidth as compared to Synchronous

Network links

Source

Target

Remote Replication


Asynchronous replication ensuring consistency
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

Remote Replication


Array based disk buffered replication
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 Data

Local Replica

Source

Remote Replica

Local Replica

Host

Remote Replication

Source Storage Array

Target Storage Array


Remote replicas tracking changes
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

Remote Replication


Array based which technology
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

Remote Replication


Three site replication
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

Remote Replication


Three site replication cascade multi hop
Three Site Replication – Cascade/Multi-hop

  • Synchronous + Disk Buffered

  • Synchronous + Asynchronous

Source Data

Remote Replica

Local Replica

Synchronous

Disk Buffered

Remote Replica

Local Replica

Bunker Site

Source Site

Remote Site

Source Data

Remote Replica

Local Replica

Synchronous

Asynchronous

Remote Replica

Local Replica

Bunker Site

Source Site

Remote Site

Remote Replication


Three site replication triangle multi target

Asynch

with

Differential

Resynch

Three Site Replication – Triangle/Multi-target

BUNKER

Sync

SAN

SOURCE

SAN

Async

SAN

REMOTE

Remote Replication


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

SAN Based Remote Replication

EMC Symmetrix

EMC CLARiiON

SAN/WAN

Hitachi

HP

IBM

Remote Replication


San based replication terminologies
SAN Based Replication: Terminologies over the SAN/WAN

  • 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

Remote Array

Control Array

SAN/WAN

C

C

PUSH

Remote Device

Control Device

PULL

Remote Replication


Network options for remote replication
Network Options for Remote Replication over the SAN/WAN

  • 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

Remote Replication


Dense wavelength division multiplexing dwdm

Optical Channels over the SAN/WAN

ESCON

Fibre Channel

Gigabit Ethernet

Optical Lambda λ

Electrical

Optical

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

Remote Replication


Synchronous optical network sonet
Synchronous Optical Network (SONET) over the SAN/WAN

  • 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

OC48

OC3

SONET

STM-16

STM-1

SDH

Remote Replication


Chapter summary
Chapter Summary over the SAN/WAN

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


Concept in practice emc remote replication
Concept in Practice – EMC Remote Replication over the SAN/WAN

  • 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

Remote Replication


Check your knowledge
Check Your Knowledge over the SAN/WAN

  • 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 chapter?

  • Discuss the effects of a bunker failure in a three-site replication for the following implementation:

    • Multihop—synchronous + disk buffered

    • Multihop—synchronous + asynchronous

    • Multitarget

Remote Replication


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