Once a rebalance process has been started the progress of the rebalance can be monitored by accessing the /pools/default/rebalanceProgress endpoint. This returns a JSON structure continaing the current progress information:

shell> curl -u admin:password 'http://Administrator:Password@192.168.0.77:8091/pools/default/rebalanceProgress'

As a pure REST API call:

GET /pools/default/rebalanceProgress HTTP/1.1
Authorization: Basic QWRtaW5pc3RyYXRvcjpUYW1zaW4=
User-Agent: curl/7.24.0 (x86_64-apple-darwin12.0) libcurl/7.24.0 OpenSSL/0.9.8r zlib/1.2.5
Host: 192.168.0.77:8091
Accept: */*

The response data packet contains a JSON structure showing the rebalance progress for each node. The progress figure is provided as a percentage (shown as a floating point value betweeen 0 and 1).

{
    "status":"running",
    "ns_1@192.168.0.56":{"progress":0.2734375},
    "ns_1@192.168.0.77":{"progress":0.09114583333333337}
}