1. Multiple Replicas. Remote Replication. DR in practice.
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
Lecture 30
Multiple Replicas. Remote Replication. DR in practice.
Chapter 3.4 – Remote Replication

2. Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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

3. What is Remote Replication?
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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
Remote Replication

4. Synchronous Replication
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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 1 4 3 2
Host
Data Write
Data Acknowledgement
Target
Remote Replication

5. Synchronous Replication: Bandwidth Requirement
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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
Required bandwidth
Max
Typical workload
Writes
MB/s
Time
Remote Replication

6. Asynchronous Replication
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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 1 2 4 3
Host
Data Write
Data Acknowledgement
Target
Remote Replication

7. Asynchronous Replication: Bandwidth Requirement
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
Asynchronous Replication: Bandwidth Requirement
Response time unaffected
Bandwidth
Need average bandwidth
Buffers
Need sufficient buffers
Can be deployed over long distances
Typical
workload
Required
bandwidth
Writes
MB/s
Average
Time
Remote Replication

8. Remote Replication Technologies
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
Remote Replication Technologies
Host based
Logical Volume Manager (LVM) based
Support both synchronous and asynchronous mode
Log Shipping
Storage Array based
Disk Buffered - Consistent PITs
Combination of Local and Remote Replication
Remote Replication

9. Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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

10. LVM Based – Advantages and Limitations
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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

11. Host Based Log Shipping
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
Host Based Log Shipping
Logs IP
Original
Offered by most database Vendors
Advantages
Minimal CPU overhead
Low bandwidth requirement
Standby Database consistent
to last applied log
Logs
Stand By
Remote Replication

12. Storage Array Based Remote Replication
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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
Source
Distance
Replica
Production Server
Remote Replication
DR Server

13. Array Based – Synchronous Replication
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
Array Based – Synchronous Replication
Network links
Source
Target
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
Remote Replication

14. Array Based – Asynchronous Replication
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
Array Based – Asynchronous Replication
Network links
Source
Target
Write is received by the source array from host/server
Source array signals write complete to host/server
Write is transmitted by source array to the target array
Target array sends acknowledgement to the source array
No impact on response time
Extended distances between arrays
Lower bandwidth as compared to Synchronous
Remote Replication

15. Asynchronous Replication: Ensuring Consistency
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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

16. Array Based – Disk Buffered Replication
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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
Source Storage Array
Remote Replication
Target Storage Array

17. Remote Replicas – Tracking Changes
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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

18. Array Based – Which Technology?
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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

19. Three Site Replication
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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

20. Three Site Replication – Cascade/Multi-hop
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
Three Site Replication – Cascade/Multi-hop
Synchronous + Disk Buffered
Synchronous + Asynchronous
Source Data
Remote Replica
Local Replica
Synchronous
Disk Buffered
Remote Replica
Local Replica
Source Site
Bunker Site
Remote Site
Source Data
Remote Replica
Local Replica
Synchronous
Asynchronous
Remote Replica
Local Replica
Source Site
Bunker Site
Remote Site
Remote Replication

21. Three Site Replication – Triangle/Multi-target
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
Three Site Replication – Triangle/Multi-target
BUNKER
Sync
Asynch
with
Differential
Resynch
SAN
SOURCE
SAN
Async
SAN
REMOTE
Remote Replication

22. SAN Based Remote Replication
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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
EMC Symmetrix
EMC CLARiiON
SAN/WAN
Hitachi HP
IBM
Remote Replication

23. SAN Based Replication: Terminologies
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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 C
SAN/WAN C
PUSH
Control Device
Remote Device
PULL
Remote Replication

24. Network Options for Remote Replication
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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
Remote Replication

25. Dense Wavelength Division Multiplexing (DWDM)
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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
Electrical
Optical Lambda λ
Remote Replication

26. Synchronous Optical Network (SONET)
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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
OC48
OC3
SONET
STM-16
STM-1
SDH
Remote Replication

27. Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
Chapter Summary
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

28. Concept in Practice – EMC Remote Replication
Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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
Remote Replication

29. Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
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 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