Consistency Groups
Ref: SE 75 SRDF CLI.pdf
Enable and disable consistency protection for SRDF/A devices
The enable action enables consistency protection for devices in SRDF/Asynchronous mode by device group or device list. If data cannot be copied from the R1 to the R2, all devices in the group will be made not ready on the links to preserve R2 data consistency.
To enable consistency protection for SRDF/A pairs in device group prod, enter: symrdf -g prod enable
To enable consistency protection for SRDF/A pairs listed in device file devfile1, enter: symrdf -file devfile1 -sid 123 -rdfg 10 enable
The disable action disables consistency protection for devices in SRDF/Asynchronous mode by device group or device list. If data cannot be copied from the R1 to the R2, then only the devices in the group that are experiencing problems will be made not ready on the links. The device state for any remaining devices in the group will remain the same.
To disable consistency protection for SRDF/A pairs in device group prod, enter: symrdf -g prod disable
To disable consistency protection for SRDF/A pairs listed in device file devfile1, enter: symrdf -file devfile1 -sid -rdfg 10 disable
To enable consistency protection for SRDF/A pairs listed in a composite group (-cg), see below:
SRDF composite groups are initially created using the symcg create command and then populated with devices and device groups. For a composite group to be enabled as an SRDF consistency group, the group must be defined as a type RDF1, RDF2, or RDF21 and must be set with the consistency protection option (-rdf_consistency).
You can enable or disable consistency protection for all the devices in a composite group. When you enable the composite group for consistency, the group is referred to as an SRDF consistency group.
Note: When consistency protection is enabled on a device, you cannot have domino effect mode turned on and vice versa.
What are the benefits of storage-based consistency groups?
- Managing all write-order dependable volumes as one entity ensures that there is restartable copy of application on the secondary system instead of a recoverable copy of data at the disaster recover (DR) site.
- Consistency groups provides the capability to create an image of one or more databases that are Database Management System (DBMS) restartable copies. It does this by temporarily holding all write I/O to the related LUNs while performing a Business Continuous Volume (BCV) split operation.
- Consistency groups ensures restartable copy of application and hence offers shorter Recovery Time Objectives (RTOs) than recoverable copies in terms of access to current data.
- Consistency groups protect against data corruption in event of partial failures.
When a composite group is enabled for consistency protection:
- Its name cannot be changed without first disabling the consistency protection. You can then re-enable it using the new name of the composite group.
- If the composite group is enabled for SRDF/A consistency protection, then the SRDF daemon will begin performing cycle switches on the SRDF groups within the composite group (or named subset in MSC mode). This means that the cycle switches for all SRDF groups will be performed at the same time. The interval between these cycle switches is determined by the smallest minimum cycle time defined on the R1 SRDF groups that are part of the composite group (or named subset). The smallest minimum cycle time supported by the SRDF daemon is 3 seconds, so this value is used if the smallest minimum cycle time across all component groups is less than 3 seconds.
- If you change the minimum cycle time for any of the R1 SRDF groups while the composite group (or named subset) is enabled for SRDF/A consistency protection, the new minimum cycle time will not take effect until you disable consistency protection and then re-enable it. If you change the minimum cycle time, the new minimum cycle time will not take effect until you disable consistency protection and then re-enable it.
- You can change its contents by doing one of the following:
• Disable consistency protection on a composite group while you add or remove devices, and then re-enable consistency protection after editing the composite group. However, this leaves the devices in the composite group unprotected during the time required to edit and then re-enable the composite group.
• For an RDF1 composite group, you can dynamically modify the composite group while maintaining consistency protection during the editing process.
"How to set "Session Consistency State" for SRDF/A."
ID: emc94696
Question: How to set "Session Consistency State" for SRDF/A.
Question: How to turn 'on' Tolerance Mode for SRDF/A.
Environment: Product: Symmetrix DMX Family
Environment: Enginuity: 5670
Environment: Enginuity: 5671
Environment: Protection: SRDF/Asynchronous (SRDF/A)
Environment: EMC SW: Solutions Enabler SYMCLI SRDF/A
Problem: # symdg show <device group name> shows
Session Consistency State : Disable <--- Tolerance mode 'ON'
Root Cause:
- Tolerance mode for SRDF/A devices allows the user to choose to keep SRDF/A active even with devices not ready on the link.
- By default, tolerance mode is set to ‘on’.
- With Tolerance mode on SRDF/A will remain active even with a device or devices not ready on the link.
- Tolerance mode will not keep SRDF/A active if all links are lost, however. It is important to note that with tolerance mode on, R2 consistency is not maintained if some SRDF/A devices go NR on the link. With Tolerance Mode set to ‘off’, if any SRDF/A backed device goes NR on the link or if all SRDF/A links are lost, SRDF/A goes to a Not Ready (NR) state and SRDF/A is dropped (drop out of asynchronous mode).
- The benefit of having tolerance mode set to ‘on’ is that it should help reduce re-sync times in cases where a drive on the R2 side fails and the R2s are not locally protected. With tolerance mode 'on', the other RDF devices stay ready on the link and continue to send data to the R2 side. This prevents all SRDF/A devices from building up invalids that will have to be copied across the link once the drive is replaced and all devices can be made ready on the link again. If this scenario occurs, the state of the SRDF/A devices will be active/inconsistent until the tracks owed have been placed in the active cycle and sent to the target. When this is done and 2 cycle switches occur, the SRDF/A devices will be active /consistent.
- It should be noted that Tolerance Mode must be set to 'off' for consistency protection.
- In output from SYMCLI, Tolerance Mode is called ‘Session Consistency State’.
Fix:
symrdf -g <device group name> disable # Command to turn 'ON' Tolerance mode and disable consistency protection.
symrdf -g <device group name> enable # Command to turn 'OFF' Tolerance mode and protect consistency of data.
Notes: "Tolerance Mode" is the HW term for "Session Consistency State".
Notes: "Symmetrix Native Ordered Writes", or SNOW, is the HW term for SRDF/A.
ING Eg:
symdg show srdf_aixgold|grep -i consist
Device Consistency State : Disabled
Device Consistency Exempt State : Disabled
RDF Pair State ( R1 <===> R2 ) : Consistent
Consistency Exempt Devices : No
Session Consistency State : Disabled
R2 Data is Consistent : True
After enabling Session Consistency:
symrdf -g srdf_aixgold enable -nop
An RDF 'Enable' operation execution is
in progress for device group 'srdf_aixgold'. Please wait...
The RDF 'Enable' operation successfully executed for
device group 'srdf_aixgold'.
symdg show srdf_aixgold|grep -i consist
Device Consistency State : Enabled
Device Consistency Exempt State : Disabled
RDF Pair State ( R1 <===> R2 ) : Consistent
Consistency Exempt Devices : No
Session Consistency State : Enabled
R2 Data is Consistent : True