IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

Chapter 6. IBM TotalStorage SAN Volume Controller attachment 177

6.2.3 DS6000 Extent Pool implications

An Extent Pool is visible to both of the two servers in the DS6000, but directly managed by

only one of them.You must define a minimum of two Extent Pools with one created for each

storage server to fully exploit the resources. There is a minimum of one Rank per Extent Pool.

Although it is possible to configure many Ranks in an Extent Pool, for SAN Volume Controller

performance optimization, we do not recommend more than one.

If you need to maximize the performance of your SAN Volume Controller configuration,

allocate each Rank to its own Extent Pool so that you configure one Rank per pool. This gives

you the ability to direct our allocations to a known location within the DS6000. Furthermore,

this configuration will help you manage and monitor the resultant logical disk performance

when required.

Multiple Ranks per Extent Pool configuration

This section shows why configuring several Ranks in an Extent Pool is not optimized for

performance in an SAN Volume Controller environment.

To clearly explain this performance limitation we can use as an example the configuration

presented in Figure 6-5 on page 178.

178 IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

Figure 6-5 Shows an example configuration that illustrates this performance limitation

In this example, an Extent Pool (Extent Pool 0) is defined on a DS6000. This Extent Pool

includes 3 Ranks of 519 GB. The overall capacity of this Extent Pool is 1.5 TB. This capacity

is available through a set of 1 GB DS6000 Extents (standard DS6000 Extent size).

In this pool of available Extents, we create one DS6000 Logical Volume called volume0,

which contains all the Extents in the Extent Pool. volume0 is 1.5 TB. Due to the DS6000

internal Logical Volume creation algorithm, the Extents from the Rank1 will be assigned, then

the Extents of the Rank2 and the Extents of the Rank3. In this case, the data stored on the

first third of the Volume0 will be physically located on Rank1, the second third on the Rank2,

and the last third on Rank3.

When volume0 is assigned to the SAN Volume Controller, the Logical Volume is identified by

the SAN Volume Controller cluster as the Managed Disk, mDiskB. mDiskB is assigned to a

Managed Disk Group, MDG0, where the SAN Volume Controller Extent size is defined as 512

MB (size can be from 16 MB to 512 MB). In this Managed Disk are also defined two others

Managed Disks, mDiskA and mDiskC. mDiskA and mDiskC are also 1.5 TB and come from

the same DS6000, but they come from different Extent Pools. These Extent Pools are

similarly configured as Extent Pool 0.