IBM manual Arrays across loops, 14 DS8000 switched loop layout

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 2-14and 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 2-14is 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.

Device adapter pair

loop 0

Server

0

device

adapter

loop 1

Fibre channel switch 1

2

Front enclosure

Rear enclosure

3

4

loop 0

Server

1

device

adapter

loop 1

There are two separate switches in each 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

36DS8000 Series: Concepts and Architecture