Issue with "Synced Flush" for Elasticsearch (ES) V 1.5.2

Folks,
I am planning to upgrade my ES from 1.5.2 to 2.3.1 and I am going through regular upgrade process. I disabled the Shard allocation and I am getting an issue when I am trying to perform the Synced Flush (POST /_flush/synced). I am getting the below error:

{
"error": "InvalidIndexNameException[[_flush] Invalid index name [flush], must not start with '']",
"status": 400
}

So, I did some research and found a work around (sort of) for this issue (from Synced Flush Causes Node to Restart) and tried to target an individual index (index name: test01) rather than all indexes and I got this error:

{
"error": "RemoteTransportException[[test001][inet[/100.82.21.7:9303]][indices:data/write/index]]; nested: InvalidTypeNameException[mapping type name [flush] can't start with '']; ",
"status": 400
}

Not sure if the ES 1.5.2 has any issues with the Synced Flush API call or if I need additional plugins involved in my current ES 1.5.2 setup!!

Any inputs will be appreciated..

Thanks
Vikram Y

I believe synced flush was introduced in Elasticsearch 1.6.0, so don't think it is available in 1.5.2, which would explain the error you are getting.

Yes Christian.. I just saw this in one of the blog post (Replicas synchronization).
Well, if thats the case do we know a best possible way to perform the upgrade to newer version of ES without the "synced flush" process? Or is there a different process of upgrading these older versions of ES..??

Please do add your points..

Thanks
Vikram Y

Have you read this blog post? Make sure that you run the migration plugin to identify potential issues if you have not already done so.

Thanks dude.. I haven'e gone through this.. Let me.. I will post questions back if I get any..

Thanks again..
Vikram Y