IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

Chapter 3. Logical configuration planning 83

Figure 3-9 Host shown with dual paths to DS6000

3.10.1 Multiple host attachment

If you need to connect a large number of hosts to the DS6000, each using multiple paths, the

eight host adapter ports that are available in the DS6000 may not be sufficient to

accommodate all the connections. The solution to this problem is the use of SAN switches or

directors to switch logical connections from multiple hosts. In a zSeries environment you will

need to select a SAN switch or director that also supports FICON.

We recommend that more than one SAN switch be provided to ensure continued availability.

For example, four of the eight fibre ports in a DS6000 could be configured to go through each

of two directors. The complete failure of either director leaves half of the paths still operating.

Multipathing software

Each host that is using more than a single path to the DS6000 requires a mechanism to allow

the attached operating system to manage multiple paths to the same device, and to also

show a preference in this routing so that I/O requests for each LUN are directed to the

preferred controller. Also, when a controller failover occurs, attached hosts that were routing

all I/O for a particular group of LUNs (LUNs on either even or odd LSSs) to a particular

controller (because it was the preferred controller) must have a mechanism to allow them to

detect that the preferred path is gone. It should then be able to re-route all I/O for those LUNs

to the alternate, previously non-preferred controller. Finally, it should be able to detect when a

controller comes back online so that I/O can now be directed back to the preferred controller

on a LUN by LUN basis (determined by which LSS a LUN is a member of). The mechanism

that will be used varies by the attached host operating system, as detailed in the next two

sections.

See 5.6, “Subsystem Device Driver (SDD) - multipathing” on page 157.

Open systems host connection

In the majority of open systems environments, IBM recommends the use of the Multipath

Subsystem Device Driver (SDD) to manage both path failover and preferred path

determination. SDD is supplied free of charge to all IBM customers who use ESS 2105, SAN

Inter-controller data path

Controller 0

I0000 I0001 I0002 I0003

DA

Even LSS

Logical

Volumes

Odd LSS

Logical

Volumes

Controller 1

I0100 I0101 I0102 I0103

DA

HBA

Dual pathed host

HBA

Host Ports Host Ports

DS6000 server enclosure