Re: [Libguestfs] FYI: perf commands I'm using to benchmark nbdcopy

> > -W out of order writes, but the manual says "This is only recommended > > for preallocated devices like host devices or other raw block > > devices" which is a very unclear recommendation to me. > > What's special about host devices versus (eg) files or > > qcow2 files which means -W wouldn't always be recommended? > > This is how RHV use qemu-img convert when copying to raw preallocated > volumes. Using -W can be up to 6x times faster. We use the same for imageio > for any type of disk. This is the reason I tested this way. > > -W is equivalent to the nbdocpy multithreaded copy using a single connection. > > qemu-img does N concurrent reads. If you don't specify -W, it writes > the data in the right order (based on offset). If a read did not > finish, the copy loops waits until the read complets before > writing. This ensure exactly one concurrent write, and it is much > slower. Thanks - interesting. Still not sure why you wouldn't want to use this flag all the time.

See also: https://lists.nongnu.org/archive/html/qemu-discuss/2021-05/msg00070.html