[Libguestfs] [PATCH 08/11] v2v: nbdkit: Add the readahead filter unconditionally if it is available.

The readahead filter is a self-configuring filter that makes sequential reads faster when the plugin is slow (and all of the plugins we use here are always slow). I observed the behaviour of the readahead filter with our qcow2 overlay when converting a guest from a vCenter source. Even when doing random reads, qemu issues 64K reads which happen to also be the minimum request size of the readahead filter, so there is no extra overhead. When doing the sequential copy the readahead filter performed better than qemu curl’s readahead because it scaled the prefetched data appropriately on long contiguous stretches and then shrunk it back to 64K around fragmented parts of the base image. --- v2v/nbdkit.ml | 19 ++++++++++++++++++- 1 file changed, 18 insertions(+), 1 deletion(-) diff --git a/v2v/nbdkit.ml b/v2v/nbdkit.ml index 7237799a7..a9f8c1aeb 100644 --- a/v2v/nbdkit.ml +++ b/v2v/nbdkit.ml @@ -49,6 +49,9 @@ type t = { (* nbdkit plugin_name --dump-plugin output. *) dump_plugin : (string * string) list; + + (* nbdkit directory containing the filters. *) + filterdir : string; } (* Check that nbdkit is available and new enough. *) @@ -105,6 +108,12 @@ let common_create plugin_name plugin_args plugin_env = error_unless_nbdkit_min_version dump_config; error_unless_nbdkit_compiled_with_selinux dump_config; + (* Get the filterdir. *) + let filterdir = + try List.assoc "filterdir" dump_config + with Not_found -> + error (f_"nbdkit --dump-config output did not contain filterdir") in + (* Get the nbdkit plugin_name --dump-plugin output, which also * checks that the plugin is available and loadable. *) @@ -133,9 +142,17 @@ let common_create plugin_name plugin_args plugin_env = if have_selinux then ( (* label the socket so qemu can open it *) add_arg "--selinux-label"; add_arg "system_u:object_r:svirt_t:s0" ); + + (* Add the readahead filter is always a win for our access patterns. + * However if it doesn't exist don't worry. + *) + if Sys.file_exists (filterdir // "nbdkit-readahead-filter.so") then ( + add_arg "--filter"; add_arg "readahead" + ); + let args = get_args () @ [ plugin_name ] @ plugin_args in - { plugin_name; args; env; dump_config; dump_plugin } + { plugin_name; args; env; dump_config; dump_plugin; filterdir } (* VDDK libraries are located under lib32/ or lib64/ relative to the * libdir. Note this is unrelated to Linux multilib or multiarch. -- 2.20.1