On Sat, 9 May 2020 at 22:02, Phil Karn <karn@xxxxxxxx> wrote: > My understanding is that large sequential writes can go directly to the > SMR areas, which is an argument for a more conventional RAID array. How > hard does btrfs try to do large sequential writes? Ok, so I had not heard of SMR before it was mentioned here and immediate read the links. It did occur to me that large sequential writes could, in theory, go straight to SMR zones but it also occurred to be that it isn't completely straight forward. 1. If the drive firmware is not declaring that the drive uses SMR, and therefore the host doesn't send a specific command to begin a sequential write, how many sectors in a row does the drive wait to receive before conclusion this is a large sequential operation? 2. What happens if the sequential operation does not begin a the start of an SMR zone? The only thing that would make it easy is if the drive had a battery-backed RAM cache at least as big as an SMR zone, ideally about twice as big, so it could accumulate the data for one zone and then start writing that while accepting data for the next. As I have no idea how big these zones are I have no idea how feasible that is.
