Caveats for using hardware-accelerated Full Copy
The following are some general caveats that Dell Technologies provides when using this feature:
- VMware publishes a KB article (318941) which details when Full Copy is not issued. Such cases include VMs that have snapshots or cloning or moving VMs between datastores with different block sizes, among others.
- One case that is not included is when the configuration parameter, disk. EnableUUID is set to TRUE on Windows VMs. VMware Tools sets this parameter, so be aware this action prevents XCOPY.
- Always set MaxTransferSize to 16 MB and use claim rules.
- Limit the number of simultaneous clones using Full Copy to three or four. This number is not a strict limitation, but Dell believes that it ensures the best performance when offloading the copy process.
- A PowerMax device that has SAN Copy, TimeFinder, and certain RecoverPoint sessions, do not support Full Copy. Any cloning or Storage vMotion operation that is run on datastores that these volumes back is automatically diverted to the default VMware software copy. The vSphere Client has no knowledge of these sessions. As such the "Hardware Accelerated" column in the vSphere Client still indicates "Supported" for these devices or datastores.
- Full Copy is not supported for use with Open Replicator. VMware reverts to software copy in these cases.
- Full Copy is not supported for use with Federated Live Migration (FLM) target devices.
- SRDF is supported with Full Copy. However, certain RDF operations, such as an RDF failover, are blocked until the PowerMax has completed copying the data from a clone or Storage vMotion.
- When the available capacity of a Storage Resource Pool (SRP) reaches the Reserved Capacity value, active or new Full Copy requests can take a long time to complete. They may cause VMware tasks to timeout. For workarounds and resolutions, see Dell KB article 503348.
- Full Copy cannot be used between devices that are presented to a host using different initiators. For example, take the following scenario:
- A host has four initiators.
- The user creates two initiator groups, each with two initiators.
- The user creates two storage groups and places the source device in one, the target device in the other.
- The user creates a single port group.
- The user creates two masking views:
- One with the source device storage group and one of the initiator groups.
- One with the other initiator group and the storage group with the target device.
- The user attempts a Storage vMotion of a VM on the source device to the target device. Full Copy is rejected, and VMware reverts to software copy.
The following are caveats for SRDF/Metro when using Full Copy. In SRDF/Metro configurations, the use of Full Copy does depend on whether the site is the site supplying the external WWN identity.
- Full Copy is not used between a non-SRDF/Metro device and an SRDF/Metro device when the device WWN is not the same as the external device WWN. Typically, but not always, the nonbiased site. Remember that even when using witness, there is a bias site. In such cases, Full Copy is only supported when operations are between SRDF/Metro devices or within a single SRDF/Metro device. Otherwise, software copy is used.
- As Full Copy is a synchronous process on SRDF/Metro devices, which ensures consistency, performance is closer to software copy than asynchronous copy.
- While an SRDF/Metro pair is in a suspended state, Full Copy reverts to asynchronous copy for the target R1. Importantly, Full Copy is still bound by the restriction that the copy must take place on the same array. For example, assume the SRDF/Metro pair AA, BB is on array 001, 002. In this configuration, device AA is the R1 on 001 and its WWN is the external identity for device BB, the R2 on 002. If the pair is suspended and the bias is switched such that BB becomes the R1, the external identity remains that of AA from array 001. In other words, it appears as a device from 001. The device, however, is presented from array 002. Full Copy is only used in operations within the device itself or between devices on array 002.
The following are caveats when using Full Copy with devices in storage groups with data reduction enabled.
- When Full Copy is used on a device in a storage group with data reduction enabled, or between storage groups with data reduction enabled, Full Copy uncompresses the data during the copy and then recompresses on the target.
- When Full Copy is used on a device in a storage group with data reduction enabled to a storage group without data reduction enabled, Full Copy uncompresses the data during the copy and leaves the data uncompressed on the target.
- When Full Copy is used on a device in a storage group without data reduction enabled to a storage group with data reduction enabled, the data remains uncompressed on the target. The data is subject to the normal data reduction algorithms.
- As Full Copy is a background process, deduplication of data, if any, may be delayed until after all tracks have been copied. Full clone copies have been shown to deduplicate at close to 100% in enabled storage groups.