sd(4): /dev/sdUp vs /dev/rsdUp performance

Hi misc@,

I originally mentioned this on my 'e2fsprogs' badblocks can't use
/dev/rsd1c device' thread, but heard crickets and thought to narrow the
scope some.

Should /dev/sdUp be notably less performant than /dev/rsdUp?

/dev/rsdUp will give me 150MB/sec in both directions. /dev/sdUp will
give me 20MB/sec writes and 60MB/sec reads.

Though on mounted FFS, /dev/sdUp seems performant.

Thanks!

-Henrich