Dell SRDF Introduction

Group properties

The properties of an SRDF group are:

• Label (name)
• A set of ports on the local array used to communicate over the SRDF links
• A set of ports on the remote array used to communicate over the SRDF links
• Local group number
• Remote group number
• One or more pairs of devices

The devices in the group share the ports and associated CPU resources of the port's directors.

Advanced properties of an SRDF group include:

• Link Limbo mode—The amount of time that the array's operating environment waits after the SRDF link goes down before updating the link's status.
• Link Domino mode—Specifies whether to force SRDF devices into the Not Ready state to the application host if, for example, host I/Os cannot be delivered across the SRDF link.
• Autolink recovery—Specifies whether SRDF automatically restores the SRDF links when they become available again after an earlier failure.
• Compression—Specifies whether to use compression when sending data over the SRDF links. Both hardware and software compression are available and can be used independently or together.
  NOTE:It is not recommended to use hardware and software compression together. It is recommended to use:

  1. Compression at the network switches if available.
  2. Hardware compression on the PowerMax where hardware I/O modules are present and network compression cannot be used.
  3. Software compression on the PowerMax where hardware I/O modules are not present and network compression cannot be used.

Types of group

There are two types of SRDF group:

• Static
• Dynamic

Static groups are defined in the local array's configuration file. Dynamic groups are defined using SRDF management tools and their properties are stored in the array's cache memory.

On arrays running PowerMaxOS or HYPERMAX OS all SRDF groups are dynamic.

Group membership

An SRDF device is a member of as many SRDF groups as there are mirrors of that device. So, in a simple, 2-site configuration (see Figure 1) that consists of R1 and R2 devices, each device is a member of one group. In a concurrent SRDF configuration (see Figure 1), the R11 device is a member of two groups, one for each R2 mirror. The R2 devices are each in a single group.