Re: [LSF/MM/BPF TOPIC] Large block for I/O

Matthew Wilcox
2023-12-22T16:06:21Z

On Fri, Dec 22, 2023 at 08:10:54AM -0700, Keith Busch wrote:
> If the host really wants to write in small granularities, then larger
> block sizes just shifts the write amplification from the device to the
> host, which seems worse than letting the device deal with it.

Maybe? I'm never sure about that. See, if the drive is actually
managing the flash in 16kB chunks internally, then the drive has to do a
RMW which is increased latency over the host just doing a 16kB write,
which can go straight to flash. Assuming the host has the whole 16kB in
memory (likely?) Of course, if you're PCIe bandwidth limited, then a
4kB write looks more attractive, but generally I think drives tend to
be IOPS limited not bandwidth limited today?