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Dell EMC Solutions Enabler 9.2 SRDF Family CLI User Guide

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Geometry Compatible Mode

GCM Rules and restrictions:

Track size for FBA devices increased from 64K in Enginuity 5876 to 128K in HYPERMAX OS. Geometry Compatibility Mode supports full SRDF functionality for devices on arrays running Enginuity 5876 with an odd number of cylinders paired with devices on arrays running HYPERMAX OS.

An array running HYPERMAX OS cannot create a device that is exactly the same size as a device with an odd number of cylinders on an array running Enginuity 5876. However, SRDF requires that R1 and R2 devices in a device pair be the same size.

HYPERMAX OS manages the device size difference automatically, using the device attribute, Geometry Compatible Mode (GCM). A device with GCM set is presented as half a cylinder smaller than its true configured size, enabling full migration functionality between HYPERMAX OS and Enginuity 5876 for SRDF. For most operations, Solutions Enabler sets it automatically when required. For example, Solutions Enabler automatically sets the GCM attribute when restoring from a physically larger R2.
NOTE:The GCM flag should be cleared on the device before mapping it to a host, otherwise, in order to clear the flag it must be unmapped from the host, which results in a data outage.

Also, the symdev, symdg, symcg, symsg commands can manually set or unset GCM for a device or group using the set/unset -gcm option. Refer to the Solutions Enabler CLI Reference Guide for more information on using these commands with the -gcm attribute.

The symrdf createpair command transparently sets/unsets the GCM attribute as part of the create pair operation, as follows:
  • Sets the GCM attribute for a target device that is configured ½ cylinder larger. The source of the copy can be:
    • A device on an array running Enginuity 5876 with an odd number of cylinders and capacity that matches the GCM size of the target device.
    • A GCM device on an array running HYPERMAX OS.
  • Unsets the GCM attribute for a target device that is configured the exact same size as the source of the copy. The source of the copy can be:
    • A source device on an array running Enginuity 5876 with even number of the cylinders and capacity that matches the size of the target device on the array running HYPERMAX OS
    • A source device on the array running HYPERMAX OS without the GCM attribute.

The symdev show, symdev list -v, symdg show ld, symdg list ld -v, sympd show, and sympd list -v commands have been enhanced to report the GCM attribute.

  • The GCM setting for a device cannot be changed if the target of the data device is already part of another replication session.
  • Do not set GCM on devices that are mounted and under Local Volume Manager (LVM) control.

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