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Local Replication
Chapter 13(9.2)
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

2. Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
Chapter Objective
After completing this chapter you will be able to:
Discuss local replication and the possible uses of local replicas
Explain consistency considerations when replicating file systems and databases
Discuss host and array based replication technologies
Functionality
Differences
Considerations
Selecting the appropriate technology
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

3. Lesson: Local Replica and Data Consistency
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Lesson: Local Replica and Data Consistency
Upon completion of this lesson, you will be able to:
Define local replication
Discuss the possible uses of local replicas
Explain replica considerations such as Recoverability and Consistency
Describe how consistency is ensured in file system and database replication
Explain Dependent write principle
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

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What is Replication
Replica - An exact copy
Replication - The process of reproducing data
Local replication - Replicating data within the same array or the same data center
REPLICATION
Source
Replica (Target)
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

5. Possible Uses of Local Replicas
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Possible Uses of Local Replicas
Alternate source for backup
Fast recovery
Decision support
Testing platform
Data Migration
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

6. Replication Considerations
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Replication Considerations
Types of Replica: choice of replica tie back into RPO
Point-in-Time (PIT)
non zero RPO
Continuous
near zero RPO
What makes a replica good
Recoverability/Re-startability
Replica should be able to restore data on the source device
Restart business operation from replica
Consistency
Ensuring consistency is primary requirement for all the replication technologies
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

7. Understanding Consistency
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Understanding Consistency
Consistency is required to ensure the usability of replica
Consistency can be achieved in various ways:
For file Systems
Offline: Un-mount file system
Online: Flush host buffers
For Databases
Offline: Shutdown database
Online: Database in hot backup mode
Dependent Write I/O Principle
By Holding I/Os
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

8. File System Consistency: Flushing Host Buffer
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File System Consistency: Flushing Host Buffer
Application
File System
Data
Sync
Daemon
Memory Buffers
Logical Volume Manager
Physical Disk Driver
Source
Replica
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

9. Database Consistency: Dependent write I/O Principle
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Database Consistency: Dependent write I/O Principle
Dependent Write: A write I/O that will not be issued by an application until a prior related write I/O has completed
A logical dependency, not a time dependency
Inherent in all Database Management Systems (DBMS)
e.g. Page (data) write is dependent write I/O based on a successful log write
Necessary for protection against local outages
Power failures create a dependent write consistent image
A Restart transforms the dependent write consistent to transitionally consistent
i.e. Committed transactions will be recovered, in-flight transactions will be discarded
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

10. Database Consistency: Dependent Write I/O
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Database Consistency: Dependent Write I/O
Source
Replica
Source
Replica 1 1 1 2 2 2 3 3 3 3 4 4 4 4 C C D
Consistent
Inconsistent
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

11. Database Consistency: Holding I/O
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Database Consistency: Holding I/O
Source
Replica 1 1 5 5 2 2 3 3 4 4
Consistent
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

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Lesson Summary
Key points covered in this lesson:
Possible uses of local replicas
Alternate source for backup
Fast recovery
Decision support
Testing platform
Data Migration
Recoverability and Consistency
File system and database replication consistency
Dependent write I/O principle
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

13. Lesson: Local Replication Technologies
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Lesson: Local Replication Technologies
Upon completion of this lesson, you will be able to:
Discuss Host and Array based local replication technologies
Options
Operation
Comparison
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

14. Local Replication Technologies
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Local Replication Technologies
Host based
Logical Volume Manager (LVM) based mirroring
File System Snapshot
Storage Array based
Full volume mirroring
Pointer based full volume replication
Pointer based virtual replication
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

15. Host Based Replication: LVM Based Mirroring
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Host Based Replication: LVM Based Mirroring
Physical Volume 1
Host
Logical Volume
Logical Volume
Physical Volume 2
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

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File System Snapshot
Pointer-based replica
Uses Copy on First Write principle
Uses bitmap and block map
Bitmap: Used to track blocks that have changed on the production/source FS after creation of snap – initially all zero
Block map: Used to indicate block address from which data is to be read when the data is accessed from the Snap FS – initially points to production/source FS
Requires a fraction of the space used by the original FS
Implemented by either FS itself or by LVM
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

17. File System Snapshots – How it Works
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File System Snapshots – How it Works
Write to Production FS
Snap FS
Metadata
Prod FS
BLK
Bit
1-0
1-0
Metadata
2-0
2-0
1 Data a
3-0
3-2
3-1
3-0
2 Data b
4-1
4-0
4-0
4-1
3 Data C c
New writes
4 Data D d
1 Nodata
1 Data d
1 no data
2 Data c
2 no data
3 no data
N Data N
4 no data
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

18. File System Snapshots – How it Works
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File System Snapshots – How it Works
Reads from snap FS
Consult the bitmap
If 0 then direct read to the production FS
If 1 then go to the block map get the block address and read data from that address
Snap FS
Metadata
Prod FS
BLK
Bit
Metadata
1-0
1-0
1 Data a
2-0
2-0
2 Data b
3-2
3-1
3 Data C
4-1
4-1
4 Data D
1 Data d
1 Nodata
2 Data c
N Data N
3 no data
4 no data
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

19. Host Based Replication: Limitations
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Host Based Replication: Limitations
LVM based replicas add overhead on host CPUs
If host volumes are already storage array LUNs then the added redundancy provided by LVM mirroring is unnecessary
The devices will have some RAID protection already
Host based replicas can be usually presented back to the same server
Keeping track of changes is a challenge after the replica has been created
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

20. Storage Array Based Local Replication
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Storage Array Based Local Replication
Replication performed by the Array Operating Environment
Replicas are on the same array
Types of array based replication
Full-volume mirroring
Pointer-based full-volume replication
Pointer-based virtual replication
Array
Source
Replica
Production Server
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT
BC Server

21. Full Volume Mirroring: Attached
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Full Volume Mirroring: Attached
Target is a full physical copy of the source device
Target is attached to the source and data from source is copied to the target
Target is unavailable while it is attached
Target device is as large as the source device
Good for full backup, decision support, development, testing and restore to last PIT
Attached
Read/Write
Not Ready
Source
Target
Array
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

22. Full Volume Mirroring: Detached
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Full Volume Mirroring: Detached
After synchronization, target can be detached from the source and made available for BC operations
PIT is determined by the time of detachment
After detachment, re-synchronization can be incremental
Detached - PIT
Read/Write
Read/Write
Source
Target
Array
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

23. Full Volume Mirroring: Source and Target Relationship
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Full Volume Mirroring: Source and Target Relationship
Attached/ Synchronization
Detached
Resynchronization
Source = Target
Source ≠ Target
Source = Target
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

24. Pointer Based Full Volume Replication
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Pointer Based Full Volume Replication
Provide full copy of source data on the target
Target device is made accessible for business operation as soon as the replication session is started
Point-in-Time is determined by time of session activation
Two modes
Copy on First Access (deferred)
Full Copy mode
Target device is at least as large as the source device
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

25. Copy on First Access Mode: Deferred Mode
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Copy on First Access Mode: Deferred Mode
Write to Source
Read/Write
Read/Write
Source
Target
Write to Target
Read/Write
Read/Write
Source
Target
Read from Target
Read/Write
Read/Write
Source
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT
Target

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Full Copy Mode
On session start, the entire contents of the Source device is copied to the Target device in the background
Most vendor implementations provide the ability to track changes:
Made to the Source or Target
Enables incremental re-synchronization
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

27. Pointer Based Virtual Replication
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Pointer Based Virtual Replication
Targets do not hold actual data, but hold pointers to where the data is located
Target requires a small fraction of the size of the source volumes
A replication session is setup between source and target devices
Target devices are accessible immediately when the session is started
At the start of the session the target device holds pointers to data on source device
Typically recommended if the changes to the source are less than 30%
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

28. Virtual Replication: Copy on First Write Example
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Virtual Replication: Copy on First Write Example
Target Virtual Device
Source
Save Location
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

29. Tracking Changes to Source and Target
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Tracking Changes to Source and Target
Changes will/can occur to the Source/Target devices after PIT has been created
How and at what level of granularity should this be tracked
Too expensive to track changes at a bit by bit level
Would require an equivalent amount of storage to keep track
Based on the vendor some level of granularity is chosen and a bit map is created (one for source and one for target)
For example one could choose 32 KB as the granularity
If any change is made to any bit on one 32KB chunk the whole chunk is flagged as changed in the bit map
For 1GB device, map would only take up 32768/8/1024 = 4KB space
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

30. Tracking Changes to Source and Target: Bitmap
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Tracking Changes to Source and Target: Bitmap
Source
At PIT
Target
Source 1 1 1
After PIT…
Target 1 1 1
For resynchronization/restore
Logical OR 1 1 1 1 1
= unchanged 1
= changed
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

31. Restore/Restart Operation
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Restore/Restart Operation
Source has a failure
Logical Corruption
Physical failure of source devices
Failure of Production server
Solution
Restore data from target to source
The restore would typically be done incrementally
Applications can be restarted even before synchronization is complete
-----OR------
Start production on target
Resolve issues with source while continuing operations on target
After issue resolution restore latest data on target to source
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

32. Restore/Restart Considerations
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Restore/Restart Considerations
Before a Restore
Stop all access to the Source and Target devices
Identify target to be used for restore
Based on RPO and Data Consistency
Perform Restore
Before starting production on target
Identify Target to be used for restart
Create a “Gold” copy of Target
As a precaution against further failures
Start production on Target
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

33. Restore/Restart Considerations (cont.)
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Restore/Restart Considerations (cont.)
Pointer based Full Volume Replicas
Restores can be performed to either the original source device or to any other device of like size
Restores to the original source could be incremental in nature
Restore to a new device would involve a full synchronization
Pointer Based virtual Replicas
Restores can be performed to the original source or to any other device of like size as long as the original source device is healthy
Target only has pointers
Pointers to source for data that has not been written to after PIT
Pointers to the “save” location for data was written after PIT
Thus to perform a restore to an alternate volume the source must be healthy to access data that has not yet been copied over to the target
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

34. Array Replicas: Which Technology?
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Array Replicas: Which Technology?
Factor
Full-volume mirroring
Pointer-based full-volume replication
Pointer-based virtual replication
Performance impact on source
No impact
CoFA mode -some impact
Full copy – no impact
High impact
Size of target
At least same as the source
Small fraction of the source
Accessibility of source for restoration
Not required
CoFA mode – required
Full copy – not required
Required
Accessibility to target
Only after synchronization and detachment from the source
Immediately accessible
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

35. Creating Multiple Replicas
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Creating Multiple Replicas
Target Devices
06:00 A.M.
Source
12:00 P.M.
Point-In-Time
06:00 P.M.
12:00 A.M.
: 12 : 01 : 02 : 03 : 04 : 05 : 06 : 07 : 08 : 09 : 10 : 11 : 12 : 01 : 02 : 03 : 04 : 05 : 06 : 07 : 08 : 09 : 10 : 11 :
A.M.
P.M.
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

36. Local Replication Management: Array Based
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Local Replication Management: Array Based
Replication management software residing on storage array
Provides an interface for easy and reliable replication management
Two types of interface:
CLI
GUI
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

37. Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
Lesson Summary
Key points covered in this lesson:
Replication technologies
Host based
LVM based mirroring
File system snapshot
Array based
Full volume mirroring
Pointer-based full volume copy
Pointer-based virtual replica
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

38. Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
Exercise 1
Click the attached file
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

39. Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
Exercise 2
Click the attached file
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

40. Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
Chapter Summary
Key points covered in this chapter:
Definition and possible use of local replicas
Consistency considerations when replicating file systems and databases
Host based replication
LVM based mirroring, File System Snapshot
Storage array based replication
Full volume mirroring, Pointer based full volume and virtual replication
Choice of technology
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

41. Concept in Practice – EMC Local Replication Solutions
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Concept in Practice – EMC Local Replication Solutions
EMC Symmetrix Arrays
EMC TimeFinder/Clone
Full volume replication
EMC TimeFinder/Mirror
Full volume mirroring
EMC TimeFinder/SNAP
Pointer based replication
EMC CLARiiON Arrays
EMC SnapView Clone
EMC SnapView Snapshot
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

42. EMC Local Replication Video
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EMC Local Replication Video
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

43. Copyright © 2009 EMC Corporation. Do not Copy - All Rights Reserved.
Check Your Knowledge
Describe the uses of a local replica in various business operations.
How can consistency be ensured when replicating a database?
What are the differences among full volume mirroring and pointer based replicas?
What is the key difference between full copy mode and deferred mode?
What are the considerations when performing restore operations for each array replication technology?
ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT

44. ISMDR:BEIT:VIII:chap9.2:Madhu N:PIIT