© 2006 emc corporation. all rights reserved. business continuity: remote replication module 4.4

© 2006 EMC Corporation. All rights reserved.

Business Continuity: Remote ReplicationBusiness Continuity: Remote Replication

Module 4.4

© 2006 EMC Corporation. All rights reserved. Business Continuity – Remote Replication - 2

Remote Replication

After completing this module, you will be able to:

Explain Remote Replication Concepts– Synchronous/Asynchronous

– Connectivity Options

Discuss Host and Array based Remote Replication Technologies– Functionality

– Differences

– Considerations

– Selecting the appropriate technology


Remote Replication Concepts

Replica is available at a remote facility– Could be a few miles away or half way around the world

– Backup and Vaulting are not considered remote replication

Synchronous Replication– Replica is identical to source at all times – Zero RPO

Asynchronous Replication– Replica is behind the source by a finite margin – Small RPO

Connectivity– Network infrastructure over which data is transported from source

site to remote site


Synchronous Replication

A write has to be secured on the remote replica and the source before it is acknowledged to the host

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

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 remote site

1

3

4

2Data Write

Data Acknowledgement

Server

Disk

Disk


Synchronous Replication

Response Time Extension– Application response time will be

extended due to synchronous replication Data must be transmitted to remote

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 for at all times

Rarely deployed beyond 200 km

Average

Time

WritesMB/s

Max


Asynchronous Replication

Write is acknowledged to host as soon as it is received by the source

Data is buffered and sent to remote– 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

3Data Write

Data Acknowledgement

Server

Disk

Disk


Asynchronous Replication

Response Time unaffected

Bandwidth– Need sufficient bandwidth on

average

Buffers– Need sufficient buffers

Can be deployed over long distances

Average

Time

WritesMB/s

Max


Remote Replication Technologies

Host based– Logical Volume Manager (LVM)

Synchronous/Asynchronous

– Log Shipping

Storage Array based– Synchronous

– Asynchronous

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


LVM Based Remote Replication

NetworkVolume Group

Physical Volume 1

Physical Volume 2

Physical Volume 3

Physical Volume 1

Physical Volume 2

Physical Volume 3

Local Site Remote Site

Volume Group

Log Log

Duplicate Volume Groups at local and remote sites

All writes to the source Volume Group are replicated to the remote Volume Group by the LVM– Synchronous or Asynchronous



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

– When the issue is resolved the queued writes are sent over to the remote

– The maximum size of the log file determines the length of outage that can be withstood

In the event of a failure at the source site, production operations can be transferred to the remote site



Advantages– Different storage arrays and RAID protection can be used at the

source and remote sites

– Standard IP network can be used for replication

– Response time issue can be eliminated with asynchronous mode, with extended RPO

Disadvantages– Extended network outages require large log files

– CPU overhead on host For maintaining and shipping log files


Host Based Log Shipping

Offered by most DB Vendors

Advantages– Minimal CPU overhead

– Low bandwidth

– Standby Database consistent

to last applied log

Original

Logs

Stand By

Logs

IP Network


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 BC operationsProduction Array Remote Array

DistanceSource Replica

Network

DR ServerProduction

Server


Array Based – Synchronous Replication

Network links

Write is received by the source array from host/server

Write is transmitted by source array to the remote array

Remote 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

Network links

Write is received by the source array from host/server

Write is transmitted by source array to the remote array

Source array signals write complete to host/server

Remote array sends acknowledgement to the source array

Source Target


Array Based – Asynchronous Replication

Ensuring Consistency– Maintain write ordering

Some vendors attach a time stamp and sequence number with each of the writes, then ship the writes to the remote array and apply the writes to the remote devices in the exact order based on the time stamp and sequence numbers

Remote array applies the writes in the exact order they were received, just like synchronous

– 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 the current buffer is closed in a consistent manner

and the buffer is switched, new writes are received in the new buffer The 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 Consistent PITs

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

RPO usually in the order of hours

Lower Bandwidth requirements

Extended distance solution


Extended Distance Consistent PIT

Create a Consistent PIT Local Replica on Source Array Create a Remote Replica of this Local Replica Optionally create another replica of the Remote replica on the

remote array if needed Repeat…as automation, link bandwidth, change rate permit

SOURCE REMOTE

Network Links

Remote Replica

Local Replica

Local Replica

Source


Synchronous + Extended Distance Consistent PIT

Synchronous replication between the Source and Bunker Site

Create consistent PIT Local Replica at bunker Create Remote Replica of bunker Local Replica Optionally create additional Local Replica at Target site

from the Remote Replica if needed Repeat…as automation, link bandwidth, change rate

permit

SOURCE REMOTEBUNKER

SyncSource

Remote Replica

Local Replica

Local Replica

Remote Replica

Network Links

Network Links


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 BC 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


Primary Site Failure – Operations at Remote Site

Remote replicas are typically not available for use while the replication session is in progress

In the event of a primary site failure the replicas have to be made accessible for use

Create a local replica of the remote devices at the remote site

Start operations at the Remote site– No remote protection while primary site issues are resolved

After issue resolution at Primary Site– Stop activities at remote site– Restore latest data from remote devices to source– Resume operations at Primary (Source) Site


Array Based – Which Technology?

Synchronous– Is a must if zero RPO is required– Need sufficient bandwidth at all times– Application response time elongation will prevent extended distance

solutions (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 or sidefile/logfile capacity

Disk Buffered Consistent PITs– Extended distance solution with RPO in the order of hours– Generally lower bandwidth than synchronous or asynchronous


Storage Array Based – Remote Replication

Network Options– Most vendors support ESCON or Fibre Channel adapters for remote

replication Can connect to any optical or IP networks with appropriate protocol

converters for extended distances DWDM SONET IP Networks

– Some Vendors have native Gigabit Ethernet adapters which allows the array to be connected directly to IP Networks without the need for protocol converters


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 (commonly referred to as a lambda or ).

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 where traffic from multiple subscribers is multiplexed together and sent out onto the 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

SONET

OC3 OC48

OC48

SDH

STM-1 STM-16

STM-16


Rated Bandwidth

Link Bandwidth Mb/s

Escon 200

Fibre Channel 1024 or 2048

Gigabit Ethernet 1024

T1 1.5

T3 45

E1 2

E3 34

OC1 51.8

OC3/STM1 155.5

OC12/STM4 622.08

OC48/STM16 2488.0


Module Summary

Key points covered in this module:

Remote Replication Concepts– Synchronous/Asynchronous

– Connectivity Options

Host and Array based Remote Replication Technologies– Functionality

– Differences

– Considerations

– Selecting the appropriate technology


Check Your Knowledge

What is a Remote Replica?

What are the possible uses of Remote Replicas?

What is the difference between Synchronous and Asynchronous Replication?

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?


Apply Your Knowledge…

Upon completion of this topic, you will be able to:

Enumerate EMC’s Remote Replication Solutions for the Symmetrix and CLARiiON arrays

Describe EMC’s SRDF/Synchronous Replication Solution

Describe EMC’s MirrorView/A Replication Solution


EMC – Remote Replication Solutions

EMC Symmetrix Arrays– EMC SRDF/Synchronous

– EMC SRDF/Asynchronous

– EMC SRDF/Automated Replication

EMC CLARiiON Arrays– EMC MirrorView/Synchronous

– EMC MirrorView/Asynchronous


EMC SRDF/Synchronous - Introduction

Array based Synchronous Remote Replication technology for EMC Symmetrix Storage Arrays– Facility for maintaining real-time physically separate mirrors of

selected volumes

SRDF/Synchronous uses special Symmetrix devices– Source arrays have SRDF R1 devices– Target arrays have SRDF R2 devices– Data written to R1 devices are replicated to R2 devices

SRDF uses dedicated channels to send data from source to target array– ESCON, Fibre Channel or Gigabit Ethernet are supported

SRDF is available in both Open Systems and Mainframe environments


SRDF Source and Target Volumes

SRDF R1 and R2 Volumes can have any local RAID Protection– E.g. Volumes could have RAID-1 or RAID-5 protection

SRDF R2 volumes are in a Read Only state when remote replication is in effect– Changes cannot be made to the R2 volumes

SRDF R2 volumes are accessed under certain circumstances– Failover – Invoked when the primary volumes become unavailable

– Split – Invoked when the R2 volumes need to be concurrently accessed for BC operations


Global Cache Director

DiskDirector (DD)

ChannelDirector (CD)


DiskDirector (DD)

Remote LinkDirector (RLD)


Symmetrix Containing Target (R2) Volumes

Target Host

3

2

Global Cache Director

DiskDirector (DD)



DiskDirector (DD)



Symmetrix Containing Source (R1) Volumes

Source Host

1 4

1. Write received by Symmetrix containing Source volume

SRDF/Synchronous

Application does not receive I/O acknowledgement until data is received and acknowledged by remote Symmetrix

Write completion time is extended - No impact on Reads

Most often used in campus solutions

4. Write complete sent to host

3. Target Symmetrix sends acknowledgement to Source

2. Source Symmetrix sends write data to Target


SRDF Operations - Failover Purpose – Make Target Volumes Read Write

Source Volume status is changed to Read Only

SRDF Link is suspended

After

RWRO

Source Volume

TargetVolume

RORW

Source Volume

TargetVolume

Before


SRDF Operations - Failback

Makes target volume Read Only, resumes link, synchronize R2 to R1, and write enables source volume

RWRO

Source Volume

TargetVolume

Before

After

RORW

Source Volume

TargetVolume

sync


SRDF Operations - Split

Enables read and write operations on both source and target volumes

Suspends replication

RORW

Source Volume

TargetVolume

Before

After

RWRW

Source Volume

TargetVolume


SRDF Operations – Establish/Restore

Establish - Resume SRDF operation retaining data from source and overwriting any changed data on target

Restore - SRDF operation retaining data on target and overwriting any changed data on source

RORW

Source Volume

TargetVolume

Establish

RORW

Source Volume

TargetVolume

Restore


EMC CLARiiON MirrorView/A Overview

Optional storage system software for remote replication on EMC CLARiiON arrays– No host cycles used for data replication

Provides a remote image for disaster recovery– Remote image updated periodically - asynchronously

– Remote image cannot be accessed by hosts while replication is active

– Snapshot of mirrored data can be host-accessible at remote site

Mirror topology (connecting primary array to secondary arrays)– Direct connect and switched FC topology supported

– WAN connectivity supported using specialized hardware


MirrorView/A Terms

Primary storage system– Holds the local image for a given mirror

Secondary storage system– Holds the remote image for a given mirror

Bidirectional mirroring– A storage system can hold local and remote images

Mirror Synchronization– Process that copies data from local image to remote image

MirrorView Fractured state– Condition when a Secondary storage system is unreachable by the

Primary storage system


MirrorView/A Configuration

MirrorView/A Setup – MirrorView/A software must be loaded on both Primary and

Secondary storage system

– Remote LUN must be exactly the same size as local LUN

– Secondary LUN does not need to be the same RAID type as Primary

– Reserved LUN Pool space must be configured

Management via Navisphere Manager and CLI


ADC

MirrorView/A – Initial Synchronization

Host

Primary Image

Secondary Image

Snapshot

RLP

A B E FE’B’ B C D E F

E

Tracking DeltaMap

Transfer DeltaMap 1 1 1 1 1 10 0 0 0 0 0

00 00 0 01 1

MAP MAP


ADC

MirrorView/A – Update

Host

Primary Image

Secondary Image

Snapshot

RLP

A FE’B’ B C D E F

Tracking DeltaMap

Transfer DeltaMap

0 00 00 1 1Transfer DeltaMap

Tracking DeltaMap 0 0 0 0 0 0

MAP MAP

A’

1

0

B’

B

E”

E’

1

E

E’


ADC

MirrorView/A –Promotion (Update Failure)

Host

Primary Image

Secondary Image

Snapshot

RLP

F’E’B’ C D E F

00 00 1Transfer DeltaMap

Tracking DeltaMap 0 0 0 0 1

MAP MAP

A’

1

0

B’

B

E”

E’

1

B

Promote Secondary

Primary Image


Consistency Groups

Group of secondary images treated as a unit

Local LUNs must all be on the same CLARiiON

Remote LUNs must all be on the same CLARiiON

Operations happen on all LUNs at the same time– Ensures a restartable image group

© 2006 emc corporation. all rights reserved. business continuity: remote replication module 4.4

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