Consistency
Srdf-Operations-white-Paper.pdf => pg 24
You can set up and enable SRDF/A such that SRDF devices of an SRDF/A device group or composite group act in unison to maintain the integrity of a database being remotely mirrored on a target Symmetrix array. If a source R1 device in the group cannot propagate data to its corresponding target R2 device, SRDF/A suspends data propagation from all R1 devices in the group, halting all data flow to the R2 targets. This suspension of all data propagation, called tripping the group, ensures a consistent R2 copy of the database up to the point in time that the group was tripped. Tripping an SRDF/A device group or composite group can occur either automatically or manually.
An automatic trip occurs when one or more R1 source devices in an enabled SRDF/A group cannot propagate data to their corresponding R2 target devices. For example:
- All RDF links between the R1 and R2 might go down for an extended period of time.
- The RDF directors on the R1 side or R2 side might fail.
A manual trip occurs when you invoke the symrdf suspend, split, or failover commands for the SRDF/A group. Tripping the group creates a consistent point-in-time R2 image from the N-2 cycle (the cycle that is two cycles behind the R1 side). The current minimum cycle time is 30 seconds.
There are two choices with a manual trip: the default or the -immediate option. By default, the SRDF/A session is dropped at the end of the current “in-process” cycle, which may cause execution time of this command to be longer but results in no invalid tracks on the R2 side. By dropping on a cycle boundary, there is no need to resolve invalid tracks when SRDF/A is resumed.
Dropping the SRDF/A session immediately may result in remote invalid tracks on both the R1 and R2 sides. The SRDF/A devices go NR on the link, and write-pending tracks are converted to invalid tracks on both the R1 and R2 sides. However, dropping a session immediately does not compromise the consistency of the data on the R2 side; SRDF/A always provides a consistent image of the data at the remote site at all times. It is only during a resynchronization operation that data consistency is not guaranteed. (It is recommended that you use BCV devices to preserve a copy of the R2 devices prior to resynchronization.)
The following command drops any SRDF/A sessions for the device group AsyncGrp. The -force option is required here to ensure that you really want to suspend SRDF/A operation.
symrdf -g AsyncGrp suspend -force
To ensure consistency protection, Asynchronous replication must be set and consistency protection enabled on the device group or composite group8 containing the SRDF/A devices. For example:
symrdf -g AsyncGrp set mode async symrdf -g AsyncGrp enable
Disabling this device group causes consistency protection to be disabled. SRDF/A devices remain ready on the RDF link and operating in async mode. Data continues to flow.
symrdf -g AsyncGrp disable
SRDF/A session status can be one of the following:
• Active with RDF consistency protection enabled
The device group or composite group has been enabled for RDF consistency. The R2 side can be either consistent or inconsistent. The device group or composite group is not considered to be consistent until the involved RDF (RA) group (or groups) are consistent and at least two cycles have passed.
• Active with RDF consistency protection disabled
The SRDF/A devices are ready or NR on the RDF link and running with SRDF/A replication.
• Inactive
The SRDF/A devices are ready on the RDF link and are working with their basic methods of replication (Synchronous, Semi-Synchronous, or Adaptive Copy).
"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 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 disable # Command to turn 'ON' Tolerance mode and disable consistency protection.
symrdf -g 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