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Chapter 4. RAS 79
4.6.6 Disk scrubbing
The DS8000 will periodically read all sectors on a disk. This is designed to occur without any
interference with application performance. If ECC-correctable bad bits are identified, the bits
are corrected immediately by the DS8000. This reduces the possibility of multiple bad bits
accumulating in a sector beyond the ability of ECC to correct them. If a sector contains data
that is beyond ECC's ability to correct, then RAID is used to regenerate the data and write a
new copy onto a spare sector of the disk. This scrubbing process applies to both array
members and spare DDMs.
4.7 Power and cooling
The DS8000 has completely redundant power and cooling. Every power supply and cooling
fan in the DS8000 operates in what is known as N+1 mode. This means that there is always
at least one more power supply, cooling fan, or battery than is required for normal operation.
In most cases this simply means duplication.
Primary power supplies
Each frame has two primary power supplies (PPS). Each PPS produces voltages for two
different areas of the machine:
208V is produced to be supplied to each I/O enclosure and each processor complex. This
voltage is placed by each supply onto two redundant power buses.
12V and 5V is produced to be supplied to the disk enclosures.
If either PPS fails, the other can continue to supply all required voltage to all power buses in
that frame. The PPS can be replaced concurrently.
Battery backup units
Each frame with I/O enclosures, or every frame if the power line disturbance feature is
installed, will have battery backup units (BBU). Each BBU can be replaced concurrently,
provided no more than one BBU is unavailable at any one time. The DS8000 BBUs have a
planned working life of at least four years.
Rack cooling fans
Each frame has a cooling fan plenum located above the disk enclosures. The fans in this
plenum draw air from the front of the DDMs and then move it out through the top of the frame.
There are multiple redundant fans in each enclosure. Each fan can be replaced concurrently.
Rack power control card (RPC)
The rack power control cards are part of the power management infrastructure of the
DS8000. There are two RPC cards for redundancy. Each card can independently control
power for the entire DS8000.
Important: It should be noted that if you install the DS8000 such that both primary power
supplies are attached to the same circuit breaker or the same switch board, then the
DS8000 will not be well protected from external power failures. This is a very common
cause of unplanned outages.