In iser we sorta rely on 4k pages so we avoid
PAGE_SIZE but rather set SIZE_4K for these sort
of things (like we did in the virt_boundary).
So you still want only 4k segments even on PPC where the PAGE_SIZE is
16k?
Yes, iSER has the "no-gaps" constraint (like nvme) and some
applications in the past were known to issue vectored IO that doesn't
necessarily align with the system PAGE_SIZE. These sort of workloads
resulted in suboptimal performance on ppc, ia64 etc (almost every I/O
had gaps).
Before the block layer was able to enforce scatterlists without gaps,
iSER used bounce buffering in order to service "gappy" I/O which was
probably a lot worse than bio splitting like the block layer is doing
today, but still we felt that having 4k segments even on larger
PAGE_SIZE systems was a decent compromise.
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