Index rebuild suspended due to IO_COMPLETION wait type
Community wiki answer:


  What is IO_COMPLETION?


This wait is associated with a sort operator that runs out of memory and has to spill to tempdb. It is not immediately obvious why rebuilding an index would involve sorting, perhaps there are some details missing from the question.


  Is there anyway to get/see an estimate?


If you happen to know how large the table is (page size specifically), that would be roughly the number of reads. If you only know the size in MB (or GB), you'd have to divide by 8192 bytes (the size of a data page) to get a rough estimate of total number of reads.

Rebuilding an index might do at least 2x this number depending on which rebuild options you choose.