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
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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
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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
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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
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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
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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
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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
Upon failure at source site, production can be transferred to target site
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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
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Offered by most database Vendors
Advantages◦ Minimal CPU overhead◦ Low bandwidth
requirement◦ Standby Database
consistent to last applied log
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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
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Source Array Target Array
DistanceSource Replica
DR ServerProduction
Server
IP/FC
Network
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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
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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
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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
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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
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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
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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
Synchronous + Asynchronous
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Bunker Site Remote Site
Local Replica Remote Replica
Local ReplicaSource Data
Synchronous
Remote Replica
Disk Buffered
Source Site
Bunker Site Remote Site
Local Replica Remote Replica
Local ReplicaSource Data
Synchronous
Remote Replica
Asynchronous
Source Site
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Asynch
with
Differential
ResynchSOURCE
REMOTE
BUNKER
Sync
Async
SAN
SAN
SAN
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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
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
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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
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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 on a single optical fiber
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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
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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
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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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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
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