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Currently, there's no easy way to tell on the client side which RPC held which extent. As part of debugging work on several different issues recently, Cray has been making use of an additional debug statement (courtesy of Xyratex) that prints out the RPC, object ID, and offset in osc_brw_prep_request.
This allows gathering some very interesting data - I'll attach a graph made with this data as an example.
This graph compares RPC # (x axis) to file position (y axis) on a client running 24 IOR jobs. It does not break down by object ID, but could additionally do so.
This visualization was very helpful in understanding some strange caching behavior, and we think this will be useful in general.
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offset_vs_rpc.png