Product manual

Managing Critical Data
77 A1 69UP Rev00 5-15
5.5 Mirroring
Mirroring provides the highest level of performance and availability for all mission
critical and business critical applications. Mirroring maintains a duplicate copy of
a logical volume on two physical disk devices. CDA 7 maintains these copies
internally by writing all modified data to both devices. The mirroring operation is
transparent to the host.
The mirroring feature designates two logical volumes residing on different physical
devices as a mirrored pair, one volume being mirror-1 and the other volume being
mirror-2. The host views the mirror-1 and mirror-2 volumes as the same logical
volume because each has the same unit address.
5.5.1 Write Operations with Mirroring
CDA 7 handles a write operation to a mirrored logical volume as a normal write
operation. The channel director presents channel end and device end (or a good
ending status) to the channel after data is written to and verified in cache. The disk
directors destage the data to each logical volume in the mirrored pair maintaining
the identical copies of data.
5.5.2 Read Operations with Mirroring
During read operations, if the data is not available in cache, CDA 7 reads the data
from the disk pointed to by its performance algorithm for best system performance.
This performance algorithm tracks path busy information as well as actuator
location and what sector is currently under the disk head in each device. If a data
check occurs on the device being read, CDA 7 automatically reads the data from
the other device.
• Interleave Service Policy shares the read operations of the mirrored pair by
reading tracks from both disk devices in an alternating method, a number of
tracks from M1, and a number of tracks from M2. Interleave is designed to
achieve maximum throughput.
• Split Service Policy differs from Interleave because read operations are
assigned to either the M1 or the M2, but not to both.
• Dynamic Mirror Service Policy (DMSP) utilizes both Interleave and Split for
maximum throughput and minimal head movement. DMSP adjusts each logical
volume dynamically based on access patterns detected.