1. Metro Mirror, Global Copy, and Global Mirror Quick Reference
Background Information
Dependent write means that the start of one write operation is dependent upon the completion of a previous write to a volume in either the same disk system or a different disk system. They are the basis for providing consistent data. Consistency means that the order of dependent writes is maintained. With consistent data at the remote site, it is possible to do a data base restart, which is much faster than data base recovery.
Asynchronous processing means the separation of data transmission from the signaling of I/O complete. The order of dependent writes is not maintained, so data is inconsistent. Manual intervention is required to create consistent data. Distance between local and remote has little impact upon the response time of the primary volume.
Synchronous processing is when the I/O complete is not returned until the update is transmitted to the remote site and acknowledged by the remote.
Recovery Time Objective (RTO) The elapsed time after an outage before the Application is back up and running. (Server/Network/Workload & Data all available again.)
Recovery Point Objective (RPO) The amount of data lost in the event of a disaster. (Transactions that had been completed but we lost as they have not been reflected at the recovery site.)
PPRC Rules
PPRC secondary volumes must be off-line to all attached systems
One-to-one PPRC volume relationship
Only IBM (DS8000/DS6000/ESS)-to-IBM supported
PPRC ports defined as SCSI-FCP
Logical paths must be established between LSSs
FCP links are bidirectional and can be shared with server data transfer
Up to 8 links per LSS
More than 8 links per physical subsystem is allowed
Up to 4 LSS secondaries can be connected to a primary LSS
A secondary LSS can be connected to as many primary LSS systems as links are available
Distance FCP links 300 KM without RPQ
Metro Mirror
Uses synchronous processing to mirror data
The sites using Metro Mirror must conform to the following conditions:
Can accept some performance impact to application write I/O
RPO=0 possible using automation
Distance within maximum limits
300 KM fcp links without RPQ
Metro Mirror provides consistency by:
Specifying consistency group on the establish path command
If an error occurs between local and remote sites, the pair will suspend with messages or SNMP alerts being issued.
Automation catches the alert and issues FREEZE/RUN commands to LSSs that have dependent data. The FREEZE command also deletes the paths from the LSS.
Writes to these volumes will receive extended long busy (CKD) or I/O queue full (Open) and dependent writes will be queued.
Once all of the LSSs have been frozen, you can issue RUN commands to them to once again allow writes. Otherwise the condition lasts 2 minutes.
This solution requires automation when consistency is desired across multiple disk systems and is highly recommended in one-to-one environments.
Planned outage for testing:
FREEZE/RUN to LSSs to suspend pairs (also deletes paths)
Failover to B volumes
FlashCopy B to C volumes
Establish paths from local to remote
Failback A to B volumes
Test on C volume
Metro Mirror, Global Copy, and Global Mirror Quick Reference
This document covers:
Dependent writes and consistency
PPRC rules
Metro Mirror
Global Copy
Global Mirror
How each solution maintains consistency
Planned and unplanned procedures for each solution
March 2007
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2. Global Mirror for zSeries (XRC)
Metro Mirror
Unplanned outage:
Automation detects problem and issues FREEZE/RUN commands to LSSs and pairs suspend (also deletes paths)
Failover to B volumes
Recovery started at remote
Local site returns
Establish paths remote to local
Failback B to A volumes
Shutdown recovery
FREEZE LSSs to suspend pairs
Failover to local
Establish paths local to remote
Failback A to B volumes
Start production
Global Copy:
Uses asynchronous processing to transmit data
Global Copy is designed for those sites that conform to the following conditions:
RPO of many hours or days
Distances that may be > 300 KM
Global Copy provides consistency by:
Quiescing application at primary
Allow the data to drain or go-to-sync
FREEZE/RUN the pairs
Consistency Group is NOT specified on establish path; therefore, extended long busy or I/O queue full not returned to server
FlashCopy B volumes to C (C can be used for disaster recovery)
Reestablish paths
Resync the pairs
Planned outage procedure same as forming consistency group
Global Copy is a wonderful tool for data migration.
Global Mirror
Uses asynchronous processing to mirror data
Global Mirror is designed for those sites that conform to the following conditions:
RPO > 0 but still very current
Distance that are greater than 300 KM
Total of 8 disk systems in any combination of primary and secondary subsystems is supported. An RPQ can be requested to add more systems to the session.
Global Mirror provides consistency by:
Master session signals form consistency group
Coordination time
Creates Change Recording (CR) bit map and during this time, write I/Os are held
Maintains order of dependent writes
Once all CR bit maps are created, writes are once again responded to
Drain Out of Sync (OOS) bit map to remote
FlashCopy® secondary (B) volume to C volume
Planned outage for testing:
Pause Global Mirror session
FREEZE/RUN to LSSs to suspend pairs
Failover to B volumes
Fast Reverse Restore (FRR) FlashCopy C>B
Wait for background copy to complete
FlashCopy B>C with the same options used during Global Mirror setup
FlashCopy B>D
Reestablish paths local to remote
Failback A>B
Resume session
Test on D
Global Mirror (continued)
Unplanned outage:
Terminate Global Mirror session if still active
Issue Failover to B volumes
Check FlashCopy status
Revert or commit
Fast Reverse Restore C>B
Wait for background copy to complete
FlashCopy B>C with same options used during initial Global Mirror setup
Recovery can now be started on the B volumes
Local site returns
Establish paths from remote to local
Failback with XD or gcp option to B volumes
Once Global Copy is in steady state , quiesce host activity
Insure that OOS=0 after quiescing host activity
Failover to A volumes with XD or gcp option
Establish paths from local to remote
Failback to A volumes with XD or gcp option
Reestablish any Master to Subordinate links that may have been lost
Start the Global Mirror session
Resume production activity to the A volumes
CHAN EXT
FC ‘C’ Target
PPRC 'A'
PRIMARY
CHAN EXT
PPRC ‘B’ Secondary and FC Source
Global Copy over long distance
FlashCopy
Summary
Global Mirror for zSeries (XRC)
Metro Mirror
Global Copy
Global Mirror
RPO
> (3-5 sec)
> 0 (hours)
> 0
(3-5 sec)
Distance
No Limit
300 KM (fcp)
No limit
Asynch
Yes No
CKD FB
CKD & FB