Write Cache Feature - NEC NF2300-SR412E User Manual

2.2

Write Cache Feature

For RAID level 5, 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.
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 power supply and battery backup unit to retain data in the
cache memory.
The backup time is restricted as follows:
• Four days in full charge status of both the two batteries in the installation of cache
memory of the maximum capacity
The following conditions are imposed to make the write cache feature effective:
• Two controllers, one or more battery backup units, and power supplies for the battery
backup unit (PS0 for BBU0, PS1 for BBU1) are installed to operate normally.
• 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.
If the battery switch on battery backup unit is not turned on, the battery backup unit does
not operate.
The cache memory is subject to battery backup by the battery backup unit.
However, for higher safety of data protection, it is recommended to use UPS
(uninterruptive power supply).
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