NEC NF1500-SR40E, User Manual

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xxvii
Write Cache Feature
For RAID type 5, 6, or 50, the performance of the disk array unit may be decreased during the writing
of a small amount of data. This is because the previous data and parity data must be read to recalculate
the parity.
This 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 cached data to the disk
drive, 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 features a battery
backup unit installed in the power supply to retain data in the cache memory.
The backup time of the 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.
Period of Service
Up to 5 years
Number of Battery Backup
Units Installed
2GB 4GB 8GB
Operating Conditions
Standard configuration
(S1500) 24H 24H –
Expanded configuration with
optional batteries
(S1500) 72H 48H –
Standard configuration
(S2500) – 72H 36H
Ambient temperature: @25ºC
Backup frequency: Once a week
16-hour discharging (power-off)
* If the ambient temperature or backup frequency exceeds the conditions described
above, the battery back units need to be replaced (not a warranty replacement) before
they reach their service life of 5 years. Ask your NEC sales representative for the
relationship between the operation mode and the life of a battery backup unit.
The following conditions are necessary to make the write cache feature effective:
„
The disk array unit will operate normally with the following components installed:
–
2 controllers (or 1 controller if you have enabled 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 after about 6 hours.