Volume Replicator 7.4.1 Administrator's Guide - Windows

Last Published:
Product(s): InfoScale & Storage Foundation (7.4.1)
Platform: Windows
  1. Understanding Volume Replicator
    1. About Volume Replicator
      1.  
        Feature highlights of Volume Replicator
      2.  
        About Volume Replicator support for IPv6 and dual stack
    2. Basic Volume Replicator terms
      1.  
        Primary and Secondary host
      2.  
        Write-order fidelity
      3.  
        Consistent data versus up-to-date data
      4.  
        Heartbeat protocol
    3. Building blocks of Volume Replicator
      1.  
        Replicated Volume Group
      2.  
        Replicator Log volume
      3.  
        Replication Link - RLINK
      4.  
        Replicated Data Set
      5.  
        Data Change Map
    4. Understanding replication in the Volume Replicator environment
      1.  
        Volume Replicator at the Primary
      2.  
        Volume Replicator at the Secondary
      3.  
        How replication happens in the Volume Replicator environment
    5. Modes of replication
      1. Synchronous mode of replication
        1.  
          Advantage of synchronous mode of replication
        2.  
          Disadvantages of synchronous mode of replication
      2.  
        Understanding data flow in Volume Replicator synchronous mode
      3. Asynchronous mode of replication
        1.  
          Advantages of Asynchronous mode of replication
        2.  
          Disadvantages of Asynchronous mode of replication
    6. Understanding data flow in Volume Replicator asynchronous mode
      1.  
        Synchronous override mode
    7.  
      Understanding data flow in an RDS that contains multiple Secondary hosts
    8. Managing data during failure and recovery
      1. Preventing data loss
        1.  
          Preventing data loss during normal operations
        2.  
          Preventing data loss during a Primary host failure
      2.  
        Maintaining data consistency
      3.  
        Detecting host and connection failures
      4.  
        Securing Volume Replicator
  2. Replication concepts
    1. About using Volume Replicator as a disaster recovery tool
      1.  
        Migrating the Primary role
      2.  
        Taking over the Primary role
      3.  
        Performing a takeover using the fast-failback option
    2. Understanding how Volume Replicator logs writes to the Replicator Log
      1. Sizing the Replicator Log
        1.  
          Determining the size of the Replicator Log
    3. Understanding replication settings for a Secondary
      1.  
        Mode of replication - synchronous attribute
      2. Using the available bandwidth effectively
        1.  
          Bandwidth of the available network connection
      3.  
        Choosing the network protocol
    4. Measures to protect log overflow and replication latency
      1. Replicator Log overflow protection - srlprot attribute
        1.  
          Replicator Log protection when Primary and Secondary are connected
        2.  
          Replicator Log protection when Primary and Secondary are disconnected
        3.  
          Changing between the states of Replicator Log protection
      2. Latency protection - latencyprot attribute
        1.  
          Understanding latency protection
        2.  
          Latency protection when Primary and Secondary are connected
        3.  
          Latency protection when Primary and Secondary are disconnected
    5. Pausing the replication
      1.  
        Pausing the replication from the Primary host
      2.  
        Pausing the replication from the Secondary host
      3.  
        Applications of the pause feature
    6.  
      Understanding checkpoints
    7. Synchronizing the Secondary
      1. Using Automatic Synchronization
        1.  
          Performing intelligent synchronization
      2.  
        Using incremental synchronization after log overflow
      3.  
        Using backup and checkpoint
    8. Understanding Volume Replicator support for FlashSnap
      1. About the snapshot operation
        1.  
          Conditions for creating disk group split friendly snapshots
        2.  
          Forcing the snapshot operation
      2.  
        About the snapback operation
    9. About Synchronized Snapshots
      1.  
        How Volume Replicator creates synchronized snapshots
    10. Understanding Bunker replication
      1.  
        About Bunker replication
      2.  
        Advantages of Bunker replication
      3.  
        How Bunker replication differs from normal replication
      4.  
        Bunker node workflow during normal operations
      5.  
        Using the Bunker node for disaster recovery
    11. Understanding Volume Replicator Support for TCP Multi-Connection
      1.  
        Advantages of TCP Multi-Connection
    12.  
      About Volume Replicator compression
    13. About Volume Replicator memory monitoring and control support
      1.  
        Advantages of memory monitoring
    14. About Volume Replicator Graphs
      1.  
        General functionality constraints for Volume Replicator Graphs in a clustered environment
  3. Setting up replication
    1.  
      About setting up replication
    2.  
      Best practices for setting up replication
    3. Security considerations for Volume Replicator
      1.  
        Configuring the VxSAS Service
      2.  
        Enabling NAT support for Volume Replicator
    4. Setting up replication using the Setup Replicated Data Set wizard
      1.  
        Prerequisites for setting up the RDS
    5.  
      Creating a Replicated Data Set (RDS)
    6.  
      Replication failover considerations with diverse network configurations
    7. Setting up the Bunker RVG for replication
      1.  
        Prerequisites for setting up Bunker RVG
      2.  
        Best practices for creating the Bunker RVG
      3.  
        Adding the Bunker RVG to the RDS
  4. Using the VEA Console for Volume Replication Operations
    1.  
      About performing Volume Replicator operations in the VEA console
    2.  
      Features of VEA console
    3.  
      Launching the VEA console
    4. Managing connections
      1.  
        Connecting to a host
      2.  
        Disconnecting from a host
      3.  
        Reconnecting hosts at startup
      4.  
        Using history to view recent connections
      5.  
        Managing favorites
      6.  
        Adding a host to the favorites
      7.  
        Removing a host from the favorites
      8.  
        Switching connections
    5. Layout of the VEA console
      1.  
        Performing tasks related to views
      2.  
        Selecting objects
      3.  
        Left pane or navigation view (tree view)
      4.  
        Right pane or details view (tabular view)
      5. Status pane
        1.  
          Console view
        2.  
          Tasks view
      6.  
        URL bar
      7. Perspectives
        1. Control Panel
          1.  
            Changing the IPv6 preference through the Control Panel
      8.  
        Menu bar and toolbar
    6. Accessing the Volume Replicator options
      1. Menu bar options
        1.  
          File menu
        2.  
          Tools menu
        3.  
          Actions menu
        4.  
          Toolbar options
    7.  
      Exiting the VEA client
  5. Monitoring replication
    1.  
      About monitoring replication
    2. Interpreting the information in the Volume Replicator views
      1.  
        Viewing all the RDSs on the host
      2. Viewing RDS information
        1.  
          RVG states
        2.  
          Replication status
      3.  
        Viewing information about the Primary RVG
      4.  
        Viewing information about the Secondary RVG
      5.  
        Viewing information about the Primary data volume
      6.  
        Viewing the Replicator Log volume information
      7.  
        Viewing information about the Secondary data volume
    3. Monitoring replication using the VEA console
      1.  
        Displaying the monitor view
      2.  
        Specifying preferences for the monitor view
      3. Interpreting the information in the monitor view
        1.  
          Configuration information
        2.  
          Log usage information
        3.  
          Obtaining replication status information
        4.  
          Obtaining statistical information
        5.  
          Interpreting error information
    4. Checking replication performance using vxrlink stats
      1.  
        Identifying the most up-to-date Secondary
    5.  
      Analyzing Volume Replicator performance
    6.  
      Monitoring alerts to interpret error conditions
    7.  
      Handling Volume Replicator events
  6. Administering Volume Replicator
    1.  
      About administering Volume Replicator
    2.  
      Modifying the configuration
    3. Adding volumes
      1.  
        Prerequisite for adding data volumes to an RDS
    4.  
      Adding a Secondary host
    5. Administering the RVG
      1.  
        Enabling or disabling data access to the RVG data volumes
      2.  
        Expanding the data volumes
      3.  
        Expanding the Replicator Log
      4. Shrinking the data volumes
        1.  
          Before shrinking a data volume
        2.  
          Shrinking a data volume
      5. Adding or removing the DCM logs from the data volumes
        1.  
          Adding or removing the DCM logs for all volumes in an RVG
      6.  
        Resynchronizing the Secondary hosts
      7. Associating or dissociating the Replicator Log volume
        1.  
          Associating the Replicator Log with the RVG
        2.  
          Dissociating the Replicator Log volume on an RVG
    6. Administering replication
      1.  
        Disabling the SwiftSync feature
      2.  
        Starting replication through the VEA console
      3.  
        Stopping replication using the VEA console
      4.  
        Changing replication settings for an RDS
      5. Managing checkpoints
        1.  
          Ending the checkpoint
      6. Pausing replication using Volume Replicator
        1.  
          Notes on pausing the replication
        2.  
          Pausing Secondary from the Primary
        3.  
          Resuming the Secondary host from Primary
        4.  
          Pausing the Secondary host from the Secondary
      7.  
        Converting the Primary to a Secondary
      8.  
        Migrating the Primary role within an RDS
      9. Creating snapshots for the data volumes
        1.  
          Reattaching the snapshots back to the original volumes
      10. Creating synchronized snapshots using the VSS Snapshot wizard
        1.  
          About snapshot naming convention on the Secondary
        2.  
          Creating synchronized snapshot sets
        3.  
          Prerequisites for creating synchronized snapshot sets
        4.  
          Creating schedules for synchronized snapshots
        5.  
          Displaying the status of the scheduled synchronized snapshot
        6.  
          Reattaching synchronized snapshots
        7.  
          Deleting a synchronized snapshot schedule
      11.  
        Recovering the RVG
      12.  
        Restoring the Secondary
      13.  
        Migrating VVR to support IPv6 or dual stack
    7. Administering Bunker replication
      1.  
        Stopping the replication
      2.  
        Pausing Secondary
      3.  
        Changing replication settings for Bunker RVG
      4. Associating or dissociating the Replicator Log
        1.  
          Dissociating the Replicator Log volume on Bunker RVG
        2.  
          Associating the Replicator Log with Bunker RVG
      5.  
        Activate Bunker
      6.  
        Deleting the Bunker Secondary
    8. Performing disaster recovery operation
      1.  
        Using the Bunker node to update the Secondary
      2.  
        Resynchronizing the original Primary when it becomes available
      3.  
        Updating the Secondary from the Bunker
      4. Taking over the Primary role using the fast-failback option
        1. Performing takeover with fast-failback
          1.  
            Prerequisites for takeover with fast-failback
        2.  
          Performing takeover without using fast-failback
        3.  
          General notes on take over operation
      5.  
        Performing takeover in a multiple Bunker setup
    9. Deleting Volume Replicator objects
      1. Removing data volumes
        1.  
          Understanding the remove data volume behavior in different scenarios
      2.  
        Deleting the replicated data set
      3.  
        Deleting the Primary RVG
      4.  
        Deleting the Secondary RVG
    10. Accessing data on Secondary host
      1.  
        Creating a mirror break-off
      2.  
        Creating snapshots
    11. Performing automated system recovery (ASR)
      1.  
        Automated system recovery (ASR) overview
      2.  
        Volume Replicator support for ASR
      3. ASR recovery process
        1.  
          Considerations when restoring a Secondary with a healthy Primary
      4.  
        Microsoft Cluster recovery
    12. Alternative methods to synchronize the Secondary faster
      1.  
        Method 1: Moving the Secondary RVG disk group on to a spare server within the same LAN as the Primary
      2. Method 2: Using snapshots for synchronizing the Secondary data volumes
        1.  
          Renaming Volumes on the Secondary host
      3. Method 3: Using mirrored plexes to synchronize the Secondary
        1.  
          Renaming mirror-breakoff volumes on the Secondary host
    13. Obtaining statistical information through Volume Replicator Graphs
      1.  
        Graph types and usage
      2. Viewing statistical information using Volume Replicator Graph
        1.  
          Viewing online bandwidth usage for an RLINK
        2.  
          Viewing Historic bandwidth usage for an RLINK
        3.  
          Viewing Volume Replicator Non-Paged Pool (NPP) Memory Graph
        4.  
          Saving an Online or Historic bandwidth usage graph
        5.  
          Re-opening a saved CSV graph file
        6.  
          Starting or stopping the Historic Bandwidth Data Collection
  7. Using the command line interface
    1.  
      About using the command line interface
    2.  
      Conventions for command line syntax
    3. Administering the RDS using the vxrds command
      1.  
        Activating the Bunker RVG
      2.  
        Creating and adding a Secondary RVG
      3.  
        Adding an existing volume to the RDS
      4.  
        Adding a Bunker node
      5.  
        Changing the host name or IP
      6.  
        Creating the Primary RVG
      7.  
        Deactivating the Bunker RVG
      8.  
        Deleting the Bunker node
      9.  
        Deleting the Secondary
      10.  
        Deleting the Primary
      11.  
        Dissociating data volumes
      12.  
        Resynchronizing a failed Primary with the new Primary
      13.  
        Converting a Primary to a Secondary
      14.  
        Migrating the Primary to a Secondary
      15.  
        Pausing replication using the vxrds pauserep command
      16.  
        Displaying the RDS
      17. Resizing the data volumes
        1.  
          Before resizing a data volume
        2.  
          Shrinking a data volume
      18.  
        Growing the Replicator Log volume
      19.  
        Resuming replication after pausing
      20.  
        Resynchronizing the Secondary
      21.  
        Setting replication attributes
      22.  
        Starting replication using the vxrds startrep command
      23.  
        Stopping replication using the vxrds stoprep command
      24.  
        Taking over the Primary role using the vxrds takeover command
      25.  
        Performing RLINK Operations using the vxrlink command
      26.  
        Associating a Secondary
      27.  
        Attaching a Secondary
      28.  
        Displaying the list of Secondary checkpoints
      29.  
        Deleting the Secondary checkpoint
      30.  
        Detaching an RLINK
      31.  
        Dissociating an RLINK
      32.  
        Creating new RLINK
      33.  
        Pausing the RLINK
      34.  
        Recovering the RLINK
      35.  
        Restoring the RLINK
      36.  
        Resuming the RLINK
      37.  
        Removing the RLINK
      38.  
        Setting the RLINK attributes
      39. Displaying the network statistics for the RLINK
        1.  
          Output values for vxrlink stats without the -e option
        2.  
          Output values for vxrlink stats with the -e option
      40.  
        Displaying the RLINK status
      41.  
        Identifying the most up-to-date Secondary
      42.  
        Verifying the RLINK
      43.  
        Starting the Historic Bandwidth Data Collection using the CLI
      44.  
        Stopping the Historic Bandwidth Data Collection using the CLI
    4. Administering the RVGs using the vxrvg command
      1.  
        Adding DCM log
      2.  
        Associating the Replicator Log volume to an RVG
      3.  
        Associating data volume with the RVG
      4.  
        Ending checkpoint
      5.  
        Starting the checkpoint
      6.  
        Deleting the RVG checkpoint
      7.  
        Displaying RVG checkpoints
      8.  
        Dissociating volumes from RVG
      9.  
        Dismounting data volumes
      10.  
        Creating new RVG
      11.  
        Converting a Secondary RVG to Primary RVG
      12.  
        Converting a Primary RVG to Secondary RVG
      13.  
        Recovering the RVG
      14.  
        Removing an RVG
      15.  
        Resynchronizing the RVG
      16.  
        Setting RVG attributes
      17.  
        Creating snapshots for data volumes in an RVG
      18.  
        Reattaching the snapshot volumes back to the data volumes in an RVG
      19.  
        Enabling data access (Starting the RVG)
      20.  
        Generating application statistics
      21.  
        Disabling data access (stopping the RVG)
    5. Displaying information using the vxprint command
      1.  
        Displaying a specific RLINK
      2.  
        Interpreting RLINK flag settings
      3. Displaying an individual RVG
        1.  
          Interpreting RVG flag settings
      4.  
        Displaying an individual data volume or Replicator Log
    6. Creating snapshots using the vxsnap command
      1.  
        Preparing volumes for snapshots
      2. Creating Synchronized Snapshots
        1.  
          About snapshot naming convention on the Secondary
      3.  
        Reattaching the Snapshots
    7.  
      Displaying memory statistics using the vxmemstat command
    8. Administering replicated volumes using the vxvol command
      1.  
        Associating a data volume with an RVG
      2.  
        Associating a volume to an RVG as a Replicator Log
      3.  
        Dissociating a volume from an RVG
    9. Displaying and changing replication ports using the vrport command
      1.  
        Displaying or setting ports for replicating data
      2.  
        Displaying or setting ports for heartbeats
      3.  
        Displaying or setting ports for vradmind
      4.  
        Displaying or setting ports for vxrsyncd
    10. Administering the RVG using the vxedit
      1.  
        Deleting the Volume Replicator objects
      2.  
        Setting the attributes
    11. Administering the RVG using the vxassist command
      1.  
        Adding a DCM log
      2.  
        Growing the volumes
      3.  
        Removing a DCM log
    12. Tuning Volume Replicator
      1.  
        Displaying the tunable values
      2.  
        Setting the tunable values
    13. Examples: Using the command line
      1.  
        Sample setup using the command line
      2. Example 1: Setting up replication using the command line interface
        1.  
          Creating the RLINK on the Primary system VVRPRI
        2.  
          Creating the Primary RVG on the Primary system VVRPRI
        3.  
          Creating the RLINK on the Secondary system VVRSEC
        4.  
          Creating the Secondary RVG on Secondary system VVRSEC
        5.  
          Attaching the RLINKs and starting replication on the Secondary
        6.  
          Attaching the RLINKs and starting replication on the Primary
      3.  
        Example 2: Setting up Bunker replication
      4. Example 3: Using Bunker node for disaster recovery
        1.  
          Updating the Secondary from the Bunker
        2.  
          Transferring the Primary role
        3.  
          Restoring the original Primary in a Bunker setup
        4.  
          Migrating the Primary role back to the original Primary
        5.  
          Recovering the original Primary during Bunker replay
        6.  
          Failing back to the original Primary
        7.  
          Restoring the Bunker setup after failback to original Primary
      5. Example 4: Using synchronized snapshots to restore data
        1.  
          Sample setup showing how to restore data using synchronized snapshots
        2.  
          Configuration Details
        3.  
          Preparing the Volumes
        4.  
          Creating snapshot volumes
        5.  
          Using the snapshots to recover the corrupted volumes
        6.  
          Restoring the original Primary
        7.  
          Migrating the Primary role back to the original Primary (failing back to the original Primary)
  8. Configuring Volume Replicator in a VCS environment
    1.  
      About configuring Volume Replicator in a VCS environment
    2. Components of a VCS cluster
      1.  
        Resources
      2.  
        Attributes
      3. Service groups
        1.  
          Application Service group
        2.  
          Replication Service group
    3. Illustrating a highly available Volume Replicator setup
      1.  
        List of agents for Volume Replicator
    4. How the agents work
      1. VvrRvg agent
        1.  
          VvrRvg agent-specific functions, state definitions, and attributes
        2.  
          Sample replicated Service group definition
        3.  
          Dependency graph
        4.  
          Resource dependencies within a replication Service group
        5.  
          Service group dependencies
      2. RVGPrimary agent
        1.  
          Typical multiple Secondary setup
        2.  
          How the agent works in a multiple Secondary setup
        3.  
          How the agent works in a Bunker set up
        4.  
          RVGPrimary agent-specific functions, state definitions, and attributes
        5.  
          Sample RVGPrimary resource definition
    5. Configuring the agents
      1.  
        About configuring the Disaster Recovery Solutions using the DR Wizard
      2.  
        Taking the application group offline on Secondary
      3.  
        Setting up replication using a virtual IP address
      4.  
        Changing the Primary and Secondary IP
      5.  
        Creating RLINKs between each pair of Secondary hosts
      6. Creating the replication service group
        1.  
          Prerequisites for creating the replication service group
    6. Working with existing replication service groups
      1.  
        Adding a new RVG resource to an existing replication Service group
      2.  
        Modifying an existing resource in the replication service group
  9. Configuring Volume Replicator with Hyper-V
    1.  
      Implementing Volume Replicator replication on Hyper-V with Microsoft failover cluster
    2.  
      Prerequisites for setting up Volume Replicator with Hyper-V
    3.  
      Configuring a virtual machine group and resource dependencies
    4. Configuring replication for the virtual machine
      1.  
        Setup 1: Replicating the System as well as Data disks
      2.  
        Setup 2: Replicating the Data disks
    5.  
      Recommendations and workarounds
  10. Advanced settings in Volume Replicator
    1.  
      About using the advanced settings in Volume Replicator
    2. Tuning the Volume Replicator memory parameters
      1. Understanding the concept of a buffer space
        1.  
          Shared memory between Volume Replicator and SFW
        2.  
          Minimum memory required by SFW and Volume Replicator
        3.  
          Size of the memory available on the Secondary
        4.  
          Readback buffer space on the Primary
      2.  
        Modifying the tunable values
    3. Understanding IBC messaging
      1.  
        Features of the IBC messaging
      2.  
        Application of IBC messaging
      3. IBC messaging commands
        1.  
          Command arguments
        2.  
          Registering an application
        3.  
          Unregistering the application
        4.  
          Sending a message
        5.  
          Receiving a message
        6.  
          Unfreezing the Secondary RVG
        7.  
          Displaying registered application names
        8.  
          Registering and sending messages
        9.  
          Registering and receiving messages
      4. Example: Using the IBC messaging facility to take snapshots
        1.  
          Sample setup showing how to take snapshots using the IBC messaging facility
  11. Troubleshooting Volume Replicator
    1.  
      About troubleshooting Volume Replicator
    2. Recommendations and checks
      1.  
        Encrypted files on replicated volumes
      2.  
        Selecting the mode of replication
      3.  
        Volume Replicator issues when Norton AntiVirus scan is performed
      4.  
        Monitor view does not display the RDS information
      5.  
        Preventing the connect problems
      6.  
        Configuration checks for RLINKS
      7.  
        Network, process, and operating system checks
      8.  
        Configuration checks for volume mappings
      9.  
        Troubleshooting Volume Replicator performance
      10.  
        Other information and checks
    3. Recovering from problems in a firewall or NAT setup
      1.  
        Errors when replicating across a firewall
    4. Recovering from problems during replication
      1.  
        Permission denied errors when performing Volume Replicator Operations
      2. Error when configuring the VxSAS Service
        1.  
          Configuring the VxSAS using the Service Control Manager
      3.  
        VEA Service is not started
      4.  
        Connecting to cluster having multiple IP addresses
      5.  
        Error when disabling data access to the RVG, creating Secondary RVG, adding volumes
      6.  
        Error when resizing volumes
      7.  
        Replica link already exists
      8.  
        Unable to perform delete RDS, add volume, delete volume
      9.  
        Removing the Replicator Log volume mirror
      10.  
        Pausing when writes are in progress
      11.  
        Unable to see volume name for associating Replicator Log
      12.  
        Unable to see the volume names for adding volumes to RDS
      13.  
        Adding logs to dissociated volumes
      14.  
        Using two commands in succession
      15.  
        Renaming dynamic disk group while importing
      16.  
        Problems when performing the snapshot operation
      17. Operation time-out errors
        1.  
          The vxrvg stop command displays operation time-out error
        2.  
          Pausing Secondary from Primary displays operation time-out error
    5. Problems when configuring Volume Replicator in a VCS environment
      1.  
        Application Service group does not failover correctly
    6. Problems when setting performance counters
      1.  
        Volume Replicator objects are not displayed
    7.  
      Ignore error when adding secondary using VEA
  12. Appendix A. Services and ports
    1.  
      InfoScale ports and services
  13. Appendix B. Using the vxrsync utility
    1.  
      About using the vxrsync utility
    2.  
      When to use vxrsync
    3. Understanding how the utility works
      1. Layout of the configuration file
        1.  
          Sample configuration file layout
      2. Using the vxrsync utility with the vxrclient component
        1.  
          vxrclient
        2.  
          vxrserver
    4.  
      Example: Using vxrsync for difference-based synchronization
  14. Appendix C. VR Advisor (VRAdvisor)
    1. Introducing Volume Replicator Advisor (VRAdvisor)
      1.  
        Overview of VRAdvisor
      2. How VRAdvisor works
        1.  
          Data collection
        2.  
          Data analysis
        3.  
          What-if analysis
    2.  
      Installing Volume Replicator Advisor (VRAdvisor)
    3.  
      Uninstalling VRAdvisor on Windows
    4. Collecting the sample of data
      1.  
        Collecting sample data using the VRAdvisor Wizard
      2.  
        Collecting the sample data using the diskStats command
    5. Analyzing the sample of data
      1. Understanding the results of the analysis
        1. Viewing the analysis results
          1.  
            Analysis graph
          2.  
            Analysis results
        2. Recalculating the analysis results
          1.  
            Applying different parameters to the existing sample of data
          2.  
            Performing What-if analysis
        3.  
          Recording and viewing the results
    6. Sizing the SRL
      1.  
        Overview
      2.  
        Peak usage constraint
      3.  
        Synchronization period constraint
      4.  
        Secondary backup constraint
      5.  
        Secondary downtime constraint
      6. Additional factors
        1.  
          Example

Readback buffer space on the Primary

Defines the amount of memory that the Primary can use to read data from the Replicator Log. The default value is 16MB. However, you can also specify any value that lies in the range from 4MB to 512MB.

vol_max_rdback_sz (READBACK_POOL_SIZE)

When a write request is made, a Volume Replicator data buffer is allocated to it. The data buffer is not released until the data has been written to the Primary Replicator Log and sent to all the Secondaries that are connected by synchronous RLINKs. When the buffer space becomes low, several effects are possible, depending on the configuration. Volume Replicator begins to free some buffers before sending the data across the asynchronous RLINKs. This frees up more space for incoming write requests so that they are not delayed. The cost is that it forces the freed requests to be read back from the Replicator Log later, when an RLINK is ready to send them. The need to perform readbacks have an effect on write latency because it makes the Replicator Log perform more non-sequential I/O. It also increases the load on the system and slows the rate at which data is sent to the Secondaries.