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Chapter 2. Components 37
the array is placed on each loop. If the disk enclosures were fully populated with DDMs, there
would be four array sites.
Figure 2-15 Array across loop
AAL benefits
AAL is used to increase performance. When the device adapter writes a stripe of data to a
RAID-5 array, it sends half of the write to each switched loop. By splitting the workload in this
manner, each loop is worked evenly, which improves performance. If RAID-10 is used, two
RAID-0 arrays are created. Each loop hosts one RAID-0 array. When servicing read I/O, half
of the reads can be sent to each loop, again improving performance by balancing workload
across loops.
DDMs
Each DDM is hot plugable and has two indicators. The green indicator shows disk activity
while the amber indicator is used with light path diagnostics to allow for easy identification and
replacement of a failed DDM.
At present the DS8000 allows the choice of three different DDM types:
73 GB, 15K RPM drive
146 GB, 10K RPM drive
300 GB, 10K RPM drive
2.5 Host adapters
The DS8000 supports two types of host adapters: ESCON and Fibre Channel/FICON. It does
not support SCSI adapters.
Server
0
device
adapter
loop 0
loop 1
Server
1
device
adapter
loop 0
loop 1
3
Fibre channel switch 1
4
There are two separate
switches in each enclosure.
2
Array site 0
Array site 1