[vlowther]

In my case, "background writes" literally means "do the io.WriteAt for this fixed-size buffer in another goroutine so that the one servicing the blob write can get on with encryption / CRC calculation / stuffing the resulting byte stream into fixed-size buffers". Handling it that way lets me keep the IO to the kernel as saturated as possible without the added schedule + mutex overhead sending stuff thru a channel incurs, while still keeping a hard upper bound on IO in flight (max semaphore weight) and write buffer allocations (sync.Pool). My fixed-size buffers are 32k, and it is a net win even there.

[Horos]

right — no variance, question was off target. worth noting though: the sema-bounded WriteAt goroutines are structurally a fan-out over homogeneous units, even if the pipeline feels linear from the blob's perspective. that's probably why the channel adds nothing — no fan-in, no aggregation, just bounded fire-and-forget.