Section 3 : Business ContinuityLecture 29

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

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

Replica - An exact copy Replication - The process of reproducing

data Local replication - Replicating data within

the same array or the same data center

Source Replica (Target)

REPLICATIONREPLICATION

Alternate source for backup◦ An alternative to doing backup on production volumes

Fast recovery◦ Provide minimal RTO (recovery time objective)

Decision support◦ Use replicas to run decision support operations such as

creating a report◦ Reduce burden on production volumes

Testing platform◦ To test critical business data or applications

Data Migration◦ Use replicas to do data migration instead of production

volumes

Types of Replica: choice of replica tie back into RPO (recovery point objective)◦ 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

Ensure data buffered in the host is properly captured on the disk when replica is created◦ Data is buffered in the host before written to disk

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

File System

Application

Memory Buffers

Logical Volume Manager

Physical Disk Driver

Data

SyncDaemon

Source Replica

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

Inconsistent Consistent

Source Replica

4 4

3 3

2 2

1 1

Source Replica

4 4

3 3

2

1

5

Source Replica

Consistent

4 4

3 3

2 2

1 1

5

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

Upon completion of this lesson, you will be able to:

Discuss Host and Array based local replication technologies◦ Options◦ Operation◦ Comparison

Host based◦ Logical Volume Manager (LVM) based replication

(LVM mirroring)◦ File System Snapshot

Storage Array based◦ Full volume mirroring◦ Pointer based full volume replication◦ Pointer based virtual replication

Host Logical Volume

Logical Volume

PhysicalVolume 1

PhysicalVolume 2

LVM based replicas add overhead on host CPUs◦ Each write is translated into two writes on the disk◦ Can degrade application performance

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

Both replica and source are stored within the same volume group◦ Replica cannot be accessed by another host◦ If server fails, both source and replica would be unavailable

Keeping track of changes on the mirrors is a challenge

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

Metadata

Write to Production FS

Prod FSMetadata

1 Data a2 Data b

Snap FS

1 Nodata

3 no data4 no data

BitBLK1-0 1-02-0 2-0

N Data N

New writes 3 Data C

2 no data

c

2 Data c

3-03-2

4 Data dD 1 no data1 Data d

4-04-13-13-04-14-0

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

MetadataSnap FS

1 Nodata2 Data c

3 no data4 no data

BitBLK1-0 1-02-03-24-1

2-03-14-1

1 Data d

Prod FSMetadata

1 Data a2 Data b

3 Data C4 Data D

N Data N

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

Production Server BC Server

Array

Source Replica

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

Source Target

Attached

Array

Read/Write Not Ready

After synchronization, target can be detached from the source and made available for BC (business continuity) operations

PIT is determined by the time of detachment

After detachment, re-synchronization can be incremental◦ Only updated blocks are resynchronized◦ Modified blocks are tracked using bitmaps

Source Target

Detached - PIT

Read/Write Read/Write

Array

Attached/Synchronization

Source = Target

Detached

Source ≠ Target

Resynchronization

Source = Target

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

Write to Source

Source Target

Read/Write Read/Write

Write to Target

Read from Target

Source Target

Source Target

Read/Write Read/Write

Read/Write Read/Write

On session start, the entire contents of the Source device is copied to the Target device in the background

If the replication session is terminated, the target will contain all the original data from the source at the PIT of activation◦ Target can be used for restore and recovery◦ In CoFA mode, the target will only have data was accessed

until termination, and therefore it cannot be used for restore and recovery

Most vendor implementations provide the ability to track changes: ◦ Made to the Source or Target ◦ Enables incremental re-synchronization

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%

Source Save Location

TargetVirtual Device

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

Source

Target

0= unchanged = changed

Logical OR

At PIT

Target

SourceAfter PIT…

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

1 0 0 1 0 1 0 0

0 0 1 1 0 0 0 1

1 0 1 1 0 1 0 1

1

For resynchronization/restore

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

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◦ Stop all access to the Source and Target devices◦ Identify Target to be used for restart

Based on RPO and Data Consistency◦ Create a “Gold” copy of Target

As a precaution against further failures◦ Start production on Target

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

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 impactHigh impact

Size of target At least same as the source At least same as the source Small fraction of the

source

Accessibility of source for restoration

Not requiredCoFA mode – required

Full copy – not requiredRequired

Accessibility to target

Only after synchronization and

detachment from the source

Immediately accessible Immediately accessible

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Source

Target Devices

Point-In-Time

Replication management software residing on storage array

Provides an interface for easy and reliable replication management

Two types of interface:◦ CLI◦ GUI

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

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

Additional Task

Research on EMC Replication Products

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?