Replication continued

1. Remote Replication

2. 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

3. 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

4. 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
1
3
4
2
Data Write
Data Acknowledgement
Host
Target
Source

5. 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
Time
Writes
MB/s
Required bandwidth
Typical workload
Max

6. 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
1
4
2
3
Data Write
Data Acknowledgement
Host
Target
Source

7. Asynchronous Replication: Bandwidth Requirement
 Response time unaffected
 Bandwidth
 Need average bandwidth
 Buffers
 Need sufficient buffers
 Can be deployed over long distances
Average
Time
Writes
MB/s
Required
bandwidth
Typical
workload

8. 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

9. 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

10. 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

11. Host Based Log Shipping
 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

12. 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
Distance
Source Replica
DR Server
Production
Server
IP/FC
Network

13. 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

14. Array Based – Asynchronous Replication
 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

15. 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

16. 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 Storage Array
Target Storage Array
Local Replica
Remote Replica
Local Replica
Source
Host
Source Data

17. 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

18. 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

19. 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

20. Three Site Replication – Cascade/Multi-hop
 Synchronous + Disk Buffered
 Synchronous + Asynchronous
Bunker Site Remote Site
Local Replica
Remote Replica
Local Replica
Source Data
Synchronous
Remote Replica
Disk Buffered
Source Site
Bunker Site
Remote Site
Local Replica
Remote Replica
Local Replica
Source Data
Synchronous
Remote Replica
Asynchronous
Source Site

21. Three Site Replication – Triangle/Multi-target
Asynch
with
Differential
Resynch
SOURCE
REMOTE
BUNKER
Sync
Async
SAN
SAN
S
A
N

22. 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
Hitachi
EMC Symmetrix
EMC CLARiiON
HP
IBM
SAN/WAN

23. 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
PUSH
PULL
Control Device Remote Device
C C
SAN/WAN

24. 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

25. 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

26. 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
SDH
STM-1 STM-16
SONET
OC3 OC48

27. 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

28. 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

29. 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
