[Libguestfs] [PATCH v2v] output: Don't use nbdkit-file-plugin cache=none when writing

nbdkit-file-plugin flag cache=none is meant to ensure that the file plugin does not pollute the page cache. However it doesn't yet work very well, especially for writing. As you can see here: https://gitlab.com/nbdkit/nbdkit/-/blob/048d5b9818c88355e596824355269773e... it is currently implemented by flushing the whole file (including parallel requests), and then evicting the data from the page cache. This implementation is naive. (See nbdcopy copy/file-ops.c for a much better implementation.) This causes quite a considerable slow down when writing to a local file (eg -os local). The penalty varies between machines and possibly kernels. On my Fedora Rawhide AMD server with 12 cores and SSDs, the slow down is under 5%. But on my RHEL 9 Intel NUC server with 2 cores and a hard disk, it makes a really huge difference, nearly doubling the total time taken. I left a note in the code that we should fix this in nbdkit and re-add this option or similar when it is working. --- output/output.ml | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/output/output.ml b/output/output.ml index 7e9765085d..c3bc4eeac7 100644 --- a/output/output.ml +++ b/output/output.ml @@ -99,7 +99,13 @@ let output_to_local_file ?(changeuid = fun f -> f ()) let cmd = if Nbdkit.version nbdkit_config >= (1, 22, 0) then ( let cmd = Nbdkit.add_arg cmd "fadvise" "sequential" in - let cmd = Nbdkit.add_arg cmd "cache" "none" in + (* nbdkit 1.28 has a very naive implementation of + * page cache eviction. We need to copy the one from + * nbdcopy copy/file-ops.c. Until then do not use + * this flag because it causes a large slow down on + * some machines. XXX + *) + (*let cmd = Nbdkit.add_arg cmd "cache" "none" in*) cmd ) else cmd in -- 2.32.0