Dell SRDF Introduction

Geometry Compatibility Mode

In Enginuity 5876, the track size of an FBA device is 64 KB, while in PowerMaxOS 5978 and HYPERMAX OS 5977 the track size is 128 KB. So an array running PowerMaxOS or HYPERMAX OS cannot create a device that is the same size as a device that has an odd number of cylinders on an array running Enginuity in a mixed SRDF configuration. However, SRDF requires that the devices in a device pair are the same size.

PowerMaxOS 5978 and HYPERMAX OS 5977 manage the difference in size automatically using the device attribute Geometry Compatibility Mode (GCM). A device with GCM set is presented as being half a cylinder smaller than its configured size. This enables full functionality in a mixed configuration for SRDF, TimeFinder, SnapVX, and TimeFinder emulations (TimeFinder Clone, TimeFinder VP Snap, and TimeFinder/Mirror) and ORS.

The GCM attribute can be set in two ways:

• Automatically on a target device when it is on an array running PowerMaxOS 10 (6079), PowerMaxOS 5978 or HYPERMAX OS 5977 and the source device is on an array running Enginuity 5876
• Manually using the Solutions Enabler CLI, Mainframe Enablers SRDF Host Component, or Unisphere

Notes:

• An SRDF configuration cannot contain a pair of arrays where one side runs PowerMaxOS 10 (6079) and the other runs Enginuity 5876. So GCM is not available and is not relevant in such configurations.
• Do not set GCM on devices that are mounted and under the control of a Local Volume Manager (LVM).
• Clear the GCM flag before mapping the device to a host. Otherwise, to clear the attribute, the device must be unmapped from the host which results in a data outage.
• The GCM setting for a device cannot be changed when the target of the data device is already part of another replication session.