[LU-16033] refine ll_use_fast_io() for stride mode

• Type: Improvement
• Resolution: Unresolved
• Priority: Major
• Assignee: Zhenyu Xu
• Reporter: Zhenyu Xu

Description

ll_use_fast_io() calculates whether current readahead window covers next_readahead + max rpc pages, if that's the case, we can issue a readahed RPC, so we'd not do fast IO because we will need a cl_io to issue the RPC, otherwise we'd return true and do fast IO w/o issueing readahead RPC.

While for stride mode, the to-be-covered pages algorithm is

max_rpc_pages * (ras->ras_stride_length + ras->ras_stride_bytes - 1) / stride_bytes

which is not so accurate.