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What is Change Notify and why is there a high response time?

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Applies to

Any SMB client and storage

Answer

  • Change Notify is a function of CIFS/SMB which sends any folder updates to the client.
    • Example: If two users have the same folder open, and one users deletes a file, a change notify update is sent to the second user of the deleted file.
    • This saves CPU cycles because the user doesn't have to refresh the whole folder if a large one, just the changes.
  • Change Notify only updates when the folder being monitored (for example when Windows Explorer is open on a share) has a file update.
  • The response time is the delta between watched folder changes.

Additional Information

  • The SMB2 Change Notify specification points to this separate specific part on waiting:
    • "Wait until the following conditions are satisfied:
      • There are one or more elements in ChangeNotifyEntry.NotifyEventList.
      • This change notification request is the oldest outstanding request on this Open. This means multiple change notification requests on the same Open are completed sequentially and in first-in-first-out (FIFO) order.
      • The operation is canceled as specified in section 2.1.5.19."

 

 

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