During 16MB bulk RPC I/O evaluation on rhel7, due to kernel BIO_MAX_PAGES (256) limit, the 16MB IO is divided into 16 1MB I/O submitting to underly block device one by one. And we found that the SFA disk driver got lots of 1MB IOs.

To optimize the performance, this patch introduces blk_plug into osd-ldiskfs when do bio, before submit IOs, it calls blk_start_plug, after submit all 16MB IOs, calls blk_finish_plug, so that the 16MB bulk IO will have more change to merge in the block evelvator scheduler layer.