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36 DS8000 Series: Concepts and Architecture
The intention is to only have four spares per DA pair, but this number may increase depending
on DDM intermix. We need to have four DDMs of the largest capacity and at least two DDMs
of the fastest RPM. If all DDMs are the same size and RPM, then four spares will be sufficient.
Arrays across loops
Each array site consists of eight DDMs. Four DDMs are taken from the front enclosure in an
enclosure pair, and four are taken from the rear enclosure in the pair. This means that when a
RAID array is created on the array site, half of the array is on each enclosure. Because the
front enclosures are on one switched loop, and the rear enclosures are on a second switched
loop, this splits the array across two loops. This is called
array across loops (AAL).
To better understand AAL refer to Figure 2-14 and Figure 2-15. To make the diagrams clearer,
only 16 DDMs are shown, eight in each disk enclosure. When fully populated, there would be
16 DDMs in each enclosure. Regardless, the diagram represents a valid configuration.
Figure 2-14 is used to depict the device adapter pair layout. One DA pair creates two switched
loops. The front enclosures populate one loop while the rear enclosures populate the other
loop. Each enclosure places two switches onto each loop. Each enclosure can hold up to 16
DDMs. DDMs are purchased in groups of 16. Half of the new DDMs go into the front
enclosure and half go into the rear enclosure.
Figure 2-14 DS8000 switched loop layout
Having established the physical layout, the diagram is now changed to reflect the layout of the
array sites, as shown in Figure 2-15 on page 37. Array site 0 in green (the darker disks) uses
the four left-hand DDMs in each enclosure. Array site 1 in yellow (the lighter disks), uses the
four right-hand DDMs in each enclosure. When an array is created on each array site, half of
Front enclosure
Rear enclosure
Device adapter pair
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