Transcript Remote Replication - Chandigarh University

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
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
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
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

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
Source
1
4
Host
2
3
Data Write
Data Acknowledgement
Target
◦ Goal is zero RPO
◦ RTO is as small as the time it takes
to start application on the target
site
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
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

Required bandwidth
Max
Typical workload
Writes
MB/s
Bandwidth
◦ To minimize impact on
response time, sufficient
bandwidth must be provided
at all times

Time
Rarely deployed beyond
200 km
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
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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
Source
1
2
Host
4
3
Data Write
Data Acknowledgement
◦ Replica will be behind the source
by a finite amount
◦ Typically configurable
Remote Replication
Target
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

Response time unaffected
Bandwidth
Typical
workload
Required
bandwidth
◦ Need average bandwidth

Buffers
◦ Need sufficient buffers

Writes
MB/s
Average
Can be deployed over long distances
Time
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
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
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
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

IP
Upon failure at source site,
production can be transferred to
target site
Remote Replication

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
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Logs
IP
Original
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Offered by most
database Vendors
Advantages
◦ Minimal CPU overhead
◦ Low bandwidth
requirement
◦ Standby Database
consistent
Logs
Stand By
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
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 Target Array
Source Array
◦ Can also be used for other business operations
IP/FC
Network
Source
Production
Server
Distance
Replica
DR Server Remote Replication
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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
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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
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
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
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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
Source
Host
Local Replica
Source Storage Array
Local Replica
Remote Replica
Target
Storage
Array
Remote
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
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 resynchronization between the source and replica
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
Synchronous

Asynchronous

Disk buffered
◦ Is a must if zero RPO is required
◦ Need sufficient bandwidth at all times
◦ Rarely above 125 miles
◦ 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
◦ Extended distance solution with RPO in the order of
hours
◦ Require lower bandwidth than synchronous or
asynchronous
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
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
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
Synchronous + Disk Buffered
Source Data
Remote Replica
Local Replica
Synchronous
Disk Buffered

Synchronous + Asynchronous
Remote Replica
Local Replica
Bunker Site
Source Site
Source Data
Remote Site
Remote Replica
Synchronous
Local Replica
Asynchronous
Remote Replica
Local Replica
Source Site
Bunker Site
Remote Site
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BUNKER
Asynch
with
Differential
Resynch
SOURCE
REMOTE
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
Replicate from one storage
array to any other storage
array over the SAN/WAN
◦ Implement tiered storage
◦ Data migration
◦ Remote vaulting
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Heterogeneous arrays
support
No impact to servers or the
LAN
EMC Symmetrix
EMC CLARiiON
SAN/WAN
Hitachi
HP
IBM
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
Control Array: Array responsible for the replication operations
◦ Control Device: Device on controlling array to/from which data is
being replicated
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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
C
Control Device
SAN/WAN
PUSH
PULL
C
Remote Device
Remote Replication
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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
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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
Optical Channels
on a single optical fiber

ESCON
Fibre Channel
Optical
Electrical
Optical
Lambda λ
Gigabit Ethernet
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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
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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
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
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
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
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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 threesite replication for the following implementation:
− Multihop—synchronous + disk buffered
− Multihop—synchronous + asynchronous
− Multitarget
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