I have a fuse filesystem with writeback cache enabled. We noticed a slow down when copying files to the fuse filesystem using cp. The slowdown seems to consistently trigger for the first copy but not later ones. Using passthrough_ll from https://github.com/libfuse/libfuse I was able to reproduce the issue. # start the fuse filesystem $ git clone https://github.com/libfuse/libfuse $ git checkout fuse-3.16.2 $ meson build && cd build && ninja $ mkdir /tmp/passthrough $ ./example/passthrough_ll -o writeback -o debug -f /tmp/passthrough In another terminal $ dd if=/dev/urandom of=/tmp/foo bs=1M count=4 # run this multiple times $ time cp /tmp/foo /tmp/passthrough/tmp/foo2 On my machine the first cp call takes between 0.4s and 1s. Repeated cp calls take 0.05s. If you wait long enough between attempts cp becomes slow again The debug logs for the slow runs say that the write size is 32k (or smaller). The fast runs have write sizes of 1M. strace says cp is doing writes in 128k blocks. I think I'm running a kernel based on 6.6.15. Is this a known issue? Is there any fix for this?
