This article describes about the Geometry Compatibility Mode (GCM) feature of VMAX3.
Let’s see some of the requirements for successful pairing of SRDF Devices:
- R1s & R2s must of like sizes.
- Devices must be of same emulation; CKD to CKD / FBA to FBA.
- Number of metas should be equal on both sides.
- Concatenated/Striped Metas must be paired to Concatenated/Striped Metas respectively.
- Stipe size of the two metas must be same.
HYPERMAX OS does not support meta devices however device pairs between non-meta devices on HYPERMAX frame and a meta device on Enginuity 5876 frame are supported. Device pairs between a non-meta device (HYPERMAX) and a meta device (Enginuity 5876) may be either striped or concatenated.
Need of GCM feature arises due to the track size implications. Previously SRDF requires R1 and R1 device to be exact same size for full SRDF functionality. R2 can be larger than R1 for migration purpose, but once R2 is used by any host, no restore to R1 is allowed since some part of the R2’s logical image of data may be above the size of R1. Attempting to restore to the R1 would leave the R1 with truncated image of data.
Basic limitation of VMAX3 is, if the VMAX device is configured with an odd number of cylinders, it is impossible to create a device of same size in VMAX3.
Symmetrix devices are configured in terms of cylinders. In all Symmetrix systems cylinder size is 15 tracks. Track size was increased gradually over the period of time from 32KB (DMX) to 128KB (VMAX3).
|Symmetrix||Track Size||Cylinder Size|
|DMX||32 KB||480 KB|
|DMX-3/VMAX||64 KB||960 KB|
|VMAX-3||128 KB||1920 KB|
Size of one Block: 512 Bytes
Number of Blocks per Track: 128( DMX-3/4,VMax) 256(VMax-3)
Number of Tracks per Cylinder: 15
Size of One Cylinder(DMX3/4,VMax): 15 Tracks x 128 blocks of 512 bytes = 960KB
Size of One Cylinder(VMax-3): 15 Tracks x 256 blocks of 512 bytes = 1920KB
As FBA track size has been doubled on VMAX3, it also doubles the capacity of 1 cylinder. Attempts to use SRDF between two VMAX generations with different FBA track sizes would raise a scenario where previous generation has devices that of a size is impossible to configure on VMAX3.
Eg: 1 Cylinder size on VMAX is 960 KB whereas 1920 KB on VMAX3.
Device with 2 cylinders on VMAX = 1920 KB can be created on VMAX3 with 1 cylinder.
Device with 3 cylinders on VMAX = 2880 KB cannot be created on VMAX3. Either 3240 KB (2 Cylinders) or 1920 KB (1 Cylinder) can be created on VMAX3.
Thus, device equal to the size of odd-cylinder capacity on VMAX cannot be created on VMAX3. This odd cylinder scenario has been addressed by the Geometry Compatibility Mode (GCM).
Frame running HYPERMAX OS cannot create a device that is exactly the same size as a device with an odd number of cylinder on frame running 5876 enginuity.
To address this, device attribute ‘GCM’ has been introduced in HYPERMAX OS. This feature is only applicable when a VMAX3 is a R2 of an SRDF transfer from previous generation system. A device with attribute set to GCM is treated as half a cylinder (960 KB) than its true configured size, enabling full functionality between HYPERMAX and previous generation systems.
The attribute can be set or unset manually using the set command in conjunction with symdev/symdg/symcg/symsg with a new -gcm option. 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. It will be set automatically as part of the symrdf createpair operation.
GCM setting for a device cannot be changed if the target of the device is already part of another session. And also device should not be mapped to set the attribute.