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464 IBM System Storage DS6000 Series: Copy Services with IBM System z
The Master Process continuously monitors and manages the entire environment at two
levels:
The Enterprise level
The VolumeSet level
The distinction between these levels is primarily the scope of actions performed at each level.
The Enterprise level represents the entire monitored environment, which consists of two or
three data processing locations containing storage managed by eRCMF. The VolumeSet
level corresponds to a set of Storage Servers volumes which must be maintained in a
consistent state and managed using the same type of copy operations. Some examples of a
VolumeSet are the volumes used by a set of applications, the volumes allocated to a set of
servers, or the members of a volume group.
Management at the Enterprise level affects the entire monitored environment. These actions
typically affect every volume of every VolumeSet being managed by eRCMF. For example,
the freeze command will cause suspension of Remote Copy pairs between the storage
subsystems on multiple sites, as shown in Figure 30-7.
eRCMF adds a central control function to coordinate the Remote Copy Freeze
function and
stops transmission of data to the remote site consistently across single/multiple frames.
When this function is centrally coordinated, data is consistent. Data consistency allows a very
fast, very consistent restart time at the secondary site.
Figure 30-7 eRCMF issues the freeze command in a disaster recovery scenario
There are two methods of maintaining consistent data:
Metro Mirror
– Use Synchronous Remote Copy to ensure that data is secure at the remote site before
signaling the host that the write is complete.
– During a disaster, you can combine the Consistency Group and Freeze functions in the
Storage Server with software, such as eRCMF, to insure that all Remote Copy pairs of
the Consistency Group are split in a manner that assures data consistency at the
remote site.
Host
eRCMF
Control
Unit
Control
Unit
Host
Control
Unit
Control
Unit
Host
Control
Unit
Control
Unit
Host
Control
Unit
Control
Unit
During
Control
Unit
Control
Unit
Control
Unit
•Both sites are suspend but in sync
•Sites are not insync, however data
to a site is consistent
Control
Unit
yNotification of mirroring
error
•eRCMF issues freeze command to all Storage
Servers
•suspends all PPRC pairs
•holds I/O until told to continue
•Based on policy, eRCMF will then allow host
operation to resume or continue to hold them
Control
Unit
Control
Unit
Control
Unit
Control
Unit
After, depending on policy:
yCU holds I/Os to
suspended device until
eRCMF can handle
condition
Data at remote site is at a “Known State”
Host
eRCMF
Control
Unit
Control
Unit
Host
Control
Unit
Control
Unit
Host
Control
Unit
Control
Unit
Host
Control
Unit
Control
Unit
During
Control
Unit
Control
Unit
Control
Unit
•Both sites are suspend but in sync
•Sites are not insync, however data
to a site is consistent
Control
Unit
yNotification of mirroring
error
•eRCMF issues freeze command to all Storage
Servers
•suspends all PPRC pairs
•holds I/O until told to continue
•Based on policy, eRCMF will then allow host
operation to resume or continue to hold them
Control
Unit
Control
Unit
Control
Unit
Control
Unit
After, depending on policy:
yCU holds I/Os to
suspended device until
eRCMF can handle
condition
Data at remote site is at a “Known State”