IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

Chapter 5. Host attachment 159

I/O operations currently in process. If multiple paths have the same load, a path is chosen

at random from those paths.

򐂰 Round robin. The path to use for each I/O operation is chosen at random from those paths

not used for the last I/O operation. If a device has only two paths, SDD alternates between

the two.

򐂰 Failover only. All I/O operations for the device are sent to the same (preferred) path until

the path fails because of I/O errors. Then an alternate path is chosen for subsequent I/O

operations.

Normally, path selection is performed on a global rotating basis; however, the same path is

used when two sequential write operations are detected.

5.6.2 Concurrent LMC load

With SDD you can concurrently install and activate the DS6000 Licensed Machine Code

(LMC) while applications continue running, if multiple paths from the server have been

configured. During the activation process, the host adapters inside the DS6000 might not

respond to host I/O requests for up to 30 seconds. SDD makes this process transparent to the

host system through its path-selection and retry algorithms.

5.6.3 Single path mode

SDD does not support concurrent download and installation of the Licensed Machine Code

(LMC) to the DS6000 if hosts are using a single-path mode. An example of a host using a

single path is shown in Figure 5-10 on page 160.

However, SDD does support a single-path Fibre Channel connection from your host system

to a DS6000. It is possible to create a volume group or a vpath device with only a single path.

Note: With a single-path connection, SDD cannot provide failure protection and load

balancing and this is not recommended.