Multiple Replicas. Remote Replication. DR in practice.

Lecture 30

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 - 2

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

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

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

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

Response time unaffected Bandwidth

◦ Need average bandwidth Buffers

◦ Need sufficient buffers Can be deployed over long distances


Average

Time

WritesMB/s

Required bandwidth

Typicalworkload

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


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

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


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

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

DistanceSource Replica

DR ServerProduction

Server

IP/FC

Network


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

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

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


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 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


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


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


Synchronous + Disk Buffered

Synchronous + Asynchronous


Bunker 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


Asynch

with

Differential

ResynchSOURCE

REMOTE

BUNKER

Sync

Async

SAN

SAN

SAN


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

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

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


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

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

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


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


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


Multiple Replicas. Remote Replication. DR in practice.

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