On 06/05/14 23:44, Christoph Hellwig wrote: > If we'd didn't clone bios the workflow would look something like this: > > - allocate new clone request in dm-mpath > - point clone->bio and clone->biotail to the original bio, zero > them out in the original request > - the low level driver does one or more calls to blk_update_request At this point, if blk_update_request is called with error code, the error is returned straight up to submitter of original bio before dm-mpath can try other paths. > until clone->bio is NULL, and then calls blk_finish_request (or > __blk_mq_end_io) once the request has been completed, > - This hands control back to dm-mpath, which can now call > __blk_mq_end_io without blk_update_request as the low level driver > took care of the bio completions. Primary reason of cloning bios was to intercept lower-layer errors in dm-mpath for path failover decision. If we only cloned requests, original bios were already completed at the time blk_finish_request was called for the clone. Other implementations were discussed back then. blk_update_request could skip completing bios until blk_finish_request is called. But it would become inefficient in the case of partial completion. A callback hook could be added to blk_update_request. However such an additional hook was considered as bad approach. -- Jun'ichi Nomura, NEC Corporation -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
