The IOBufferBlock _end and _start should never be reset to free the buffer space. The block itself should be released and a new one allocated to store additional information. The buffer block chain can be of arbitrary length, I haven't seen anywhere in the code that a max length is presumed. Could you point out that code? The IOBufferBlocks are treated as write once, stable blocks of memory so they can be passed along without copying the memory as the data flows through.
In general would should happen is MIOBuffer::write_avail() should be called and that will add a new block (if needed) to the writer block chain (see MIOBuffer::write_avail in iocore/eventsystem/P_IOBuffer.h). Unfortunately I don't see where that is being done, but we have run 6.2.x in production for testing and the TLS works. I'll see what I can find.
