IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

Chapter 3. Logical configuration planning 81

The maximum number of logical disks an DS6000 can support is 8192. In the DS6000, each

LSS supports a maximum of 256 devices, so if you use all 32 LSSs, the maximum number of

logical disks supported is 32 x 256 = 8192.

The DS6000 supports a maximum of 256 host login IDs per Fibre Channel/FICON host

adapter port, with a total number of host logins of 1024 per DS6000, in contrast to the ESS

800 which supports up to 128 host logins per adapter port and a maximum of 512 host logins

per ESS and the DS8000, which supports up to 509 host logins per adapter port, and a

maximum of 8192 user logins per DS8000.

iSeries users are reminded that there is a maximum of 32 logical disks supported on each

Fibre Channel adapter in an iSeries server.

These numbers are important when considering the implementation of DS6000 Copy

Services, the maximum number of hosts to attach to a given DS6000, and the number of

logical disks to assign to each host.

When considering which logical disk size to use, it is also important to consider that the

DS6000 attachment type a host uses will limit the number of logical disks that can be

presented to the host. (Typically, a Fibre Channel (SCSI) attached host operating system can

support 8 or 32 logical disks). The limit for FCP attached hosts is typically 256 logical disks.

3.10 Configuring I/O ports

Although each Host Adapter (HA) port can reach any logical volume in the disk subsystem,

Figure 3-8 on page 82 indicates a server affinity to its local HA and its Fibre Channel ports.

This introduces the concept of a preferred path. When a volume has an affinity, for example,

to server0, and is accessed through a port in the HA of server0, then the I/O is locally

processed. When this volume is accessed through the HA of the other server, in this example

from server1, then the I/O is routed across the interconnect to the server which owns the

extent pool, which here is server0.

There is a performance penalty if data from a logical volume managed by one server is

accessed from a port that is located on the other server. The request for the logical volume

and the data would have to be transferred across the bridge interface that connects both

servers. These transfers add some latency to the response time. Furthermore, this interface

is also used to mirror the persistent memory and for other inter-server communication. It

could become a bottleneck if too many normal I/O requests ran across it, although it is a high

bandwidth, low latency connection.

82 IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

Figure 3-8 Host Adapter server affinity

If you need more than two paths from a host to the DS6000, spread the attached host I/O

paths evenly between the two sets of HAs on the DS6000 servers. This will ensure that you

achieve good aggregate I/O bandwidth, and that the host retains adequate access to its data

We recommend the inclusion of some supported multipath management software such as the

Multipath Subsystem Device Driver which is included with each DS6000, and is discussed

briefly below in “Multipathing software” on page 83.

For best reliability and performance, it is recommended that each attached host has two

connections, one to each controller as depicted in Figure 3-9 on page 83. This allows it to

maintain connection to the DS6000 through both a controller failure and Host Adapter (HBA

or HA) failure.

Card 1

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Device adapter

2 Gbps Host Fibre Channel ports

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Fibre Channel switch

Fibre Channel switch

Server

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First Expansion enclosure

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