[LU-12533] Improve readahead RPC issuance for large window sizes

• Type: Improvement
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.13.0, Lustre 2.12.4
• Assignee: Patrick Farrell (Inactive)
• Reporter: Patrick Farrell (Inactive)

Description

Currently, when the Lustre readahead window crosses a stripe boundary, the lov_io_submit code stops there and sends an RPC.  This means that for striped files & large readahead windows, we will send RPCs sized, at maximum, stripe_size.

This is unnecessary - If a readahead window hits the same stripe more than once, we can take advantage of that fact and submit larger RPCs.  This is particularly apparent in strided readahead.