IBM System Storage DS3000, Programming Manual

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Table 8-1. Maximum Number of Defined Mirrors (continued)
Controller Model
Maximum Number of Defined
Mirrors
DS4800 128
DS5020 64
DS5100 128
DS5300 128
The primary logical drive is the logical drive that accepts host I/O activity and stores application data.
When the mirror relationship is first created, data from the primary logical drive is copied in its entirety
to the secondary logical drive. This process is known as a
full synchronization and is directed by the
controller owner of the primary logical drive. During a full synchronization, the primary logical drive
remains fully accessible for all normal I/O operations.
The controller owner of the primary logical drive initiates remote writes to the secondary logical drive to
keep the data on the two logical drives synchronized.
The secondary logical drive maintains a mirror (or copy) of the data on its associated primary logical
drive. The controller owner of the secondary logical drive receives remote writes from the controller
owner of the primary logical drive but will not accept host write requests. Hosts are able to read from the
secondary logical drive, which appears as read-only.
In the event of a disaster or a catastrophic failure at the primary site, you can perform a role reversal to
promote the secondary logical drive to a primary role. Hosts then are able to read from and write to the
newly promoted logical drive, and business operations can continue.
Mirror Repository Logical Drives
A mirror repository logical drive is a special logical drive in the storage subsystem that is created as a
resource for the controller owner of the primary logical drive in a remote-mirror pair. The controller
stores mirroring information on this logical drive, including information about remote writes that are not
yet complete. The controller can use this information to recover from controller resets and the accidental
powering down of the storage subsystems.
When you activate the Enhanced Remote Mirroring premium feature on the storage subsystem, you
create two mirror repository logical drives, one for each controller in the storage subsystem. An
individual mirror repository logical drive is not needed for each remote mirror.
When you create the mirror repository logical drives, you specify the location of the logical drives. You
can either use existing free capacity, or you can create an subsystem for the logical drives from
unconfigured capacity and then specify the RAID level.
Because of the critical nature of the data being stored, do not use RAID 0 as the RAID level of mirror
repository logical drives. The required size of each logical drive is 128 MB, or 256 MB total for both
mirror repository logical drives of a dual-controller storage subsystem. In previous versions of the
Enhanced Remote Mirroring feature, the mirror repository logical drives required less disk storage space
and needed to be upgraded to use the maximum amount of mirror relationships.
Mirror Relationships
Before you create a mirror relationship, you must enable the Enhanced Remote Mirroring premium
feature on both the primary storage subsystem and the secondary storage subsystem. You must also
8-2
IBM System Storage DS3000, DS4000, and DS5000: Command Line Interface and Script Commands Programming Guide