Write Cache Feature
With RAID level 5, the performance of the DAU may be decreased somewhat when
writing small amounts of data to a disk. To increase performance, the DAU is equipped
with cache memory. When write data is stored in cache memory, the DAU may
terminate command processing and then write data to disks for improved performance.
However, if the power is disrupted before the data in cache memory is written to disk,
the data in the cache memory could be lost. To prevent this, the DAU power supplies
each include a battery backup unit to supply power to the cache memory, thereby
retaining data in the cache memory should the power fail. The battery backup unit is
fully charged after eight hours of operation.
Cross Call Feature
By using the cross call feature, logical disk drives can be accessed from alternative
paths within the DAU, even if a controller fails. The cross call feature minimizes server
downtime thereby enhancing system performance. The cross call feature of the DAU
can be set with management software. The cross call feature has two states:
Figure 1-2 illustrates a system connected to the DAU with the cross call feature turned
on and a failure with DAU controller HD#1.
1-6 Disk Array Unit Overview
Off state In this state, the DAU is accessible only from the host adapter (HBA) of
the bound host.
On state (shipping default) In this state, all logical drive numbers (LDNs) in the
DAU are accessible from both controllers or one controller should one of the
Since one LDN is recognized from each controller, this
cross call must be used on the system where the logical disks can be
controlled exclusively. When you want to use the alternative path
switching feature of the host, the cross call feature must be turned
Figure 1-2. Cross Call Feature Connections