2-3
2.2
Write Cache Feature
For RAID type 5, 6, or 50, the performance of the disk array unit may be decreased during writing of a
small amount of data. It is because the previous data and parity data must be read to recalculate the
parity.
The disk array unit is equipped with cache memory. When write data is stored in the cache memory,
the disk array unit terminates the command processing and then writes the data to disk drive for
improving the performance.
The cache memory is backed up with the battery backup unit. To increase data security,
an uninterruptible power supply (UPS) should also be used.
In general, if the power is shut down before the data saved in the cache memory is written to disk
drive, the data in the cache memory will be lost. To prevent this, the disk array unit provides the
battery backup by battery backup unit installed in the power supply to retain data in the cache memory.
The backup time using the battery backup unit is restricted as follows:
The backup time of battery backup unit depends on the number of battery backup units installed. The
following table shows the backup time when the battery backup units are installed and they are fully
charged.
Number of Battery Backup
Units Installed
Period of Service
Operating Conditions
Up to 5 years
Standard configuration
(Optional batteries not
provided)
24H
8-hour charging (power-on) and
16-hour discharging (power-off) in
environment in which the ambient
temperature is @25
o
C
Expansion configuration with
optional batteries
(Optional batteries provided)
72H
* If the discharging (power-off) time exceeds 16 hours in daily use, the battery back units
need to be replaced at a cost before they reach the service life of 5 years. Ask your
NEC sales representative for the relation between the operation mode and the life of a
battery backup unit.
The following conditions are imposed to make the write cache feature effective:
The disk array unit is operating normally with the following components being installed:
–
2 controllers (or 1 controller if you have made the setting to enable Write Cache from the
maintenance PC)
–
1 or more power supplies
–
1 or more battery backup units installed in the power supply or power supplies above
(BBU0 for PS0 and BBU1 for PS1)
The battery backup units are fully charged.
If any of the conditions is not satisfied, the write cache feature does not work sufficiently.
The batteries installed in the battery backup unit are fully charged for about eight hours.
