PowerMax and VMware Storage vMotion

• Storage is PowerMax
• Storage vMotion is in use.
• vMotion may be copying more slowly than expected
• Host may be logging vMotion-related errors.

vMotion is a VMware product that uses VAAI XCOPY primitives to offload copy requests to storage, which can conserve host resources and speed up data transfers.

The process can be summarized as follows:

1. vMotion is told to move more than 256 KB of data from one location to another.
2. vMotion sends copy requests to PowerMax.
   • The total data to be moved is split into chunks by vMotion.
   • PowerMax allows requests to move up to 16 MB per extent, but vMotion defaults to chunks of 4 MB. This can be increased to a maximum of 16 MB using Claim Rules.
3. PowerMax creates an XCOPY session between the source and target device (which can be the same device).
4. PowerMax groups the chunks into "extents," and places each extent into an XCOPY table.
5. PowerMax acknowledges receipt of this chunk to vMotion.
   • Here, vMotion believes the copy to be complete.
   • Steps 4-5 are repeated for each copy request from vMotion.
   • If PowerMax sees a problem with a copy request, the request is rejected. vMotion defaults to copying the chunk using normal host reads or writes if none of the retries were successful.
6. PowerMax works through the XCOPY table, moving the data from source to target asynchronously from the vMotion copy request.
   • Read or write functions within PowerMax perform copy.
   • This should complete quickly, but may copy more slowly if high resources are needed for ongoing host I/O.

7. VMware releases the data from the copy request for host use at the target location.
8. If the initial vMotion operation was a move rather than a copy and UNMAP is enabled, the host issues UNMAP commands to the source track outside of the vMotion protocol.

VMware details the requirements for use of Storage vMotion with VAAI XCOPY in their article VMware vSphere APIs: Array Integration (VAAI) .

PowerMax requirements for use of vMotion:

• Both source and target devices must not be involved in SRDF/Metro (active/active)
• Both source and target devices must not be involved in SnapVX sessions. 

As stated in step 5 of the Cause section above, PowerMax rejects copy requests if the request is not supported. Common rejection reasons include:

• Unable to Cascade
  • Data copy to the first target must be complete before that same data can be copied from the other target. If the first copy is not yet complete (still in the XCOPY table), a copy request for any portion of that same data to another location is rejected.
• Unaligned Extents: 
  • The logical block address (LBA) for the beginning and end of the chunk must be at the first block of a 128 KB track, and the LBA at the end of the chunk must be at the last block of a 128 KB track. This must be true at both the source and the target for the copy request to be accepted.
  • The copy request must contain real data. If the request contains only extents with block counts of zero, it is rejected.
  • The copy request must contain at least 256 (0x100) blocks of data within an extent. Smaller data amounts are rejected.

If you need further assistance troubleshooting the issues above, contact Dell Technical Support or your Authorized Service Representative and provide this knowledge article number.