On RHEL 7.5 we need to include <linux/falloc.h> for FALLOC_FL_* macros. Without the macros, fallocate is never used and we fall back to manual zeroing. Here are examples runs with this change with a local file on ext4: $ export SOCK=/tmp/nbd.sock $ export FILE=/var/tmp/nbd.img $ export BLOCK=/dev/loop2 $ src/nbdkit -f plugins/file/.libs/nbdkit-file-plugin.so file=$FILE -U $SOCK $ time qemu-img convert -n -f raw -O raw /var/tmp/fedora-27.img nbd:unix:$SOCK real 0m13.361s user 0m0.127s sys 0m0.668s $ src/nbdkit -f plugins/file/.libs/nbdkit-file-plugin.so file=$BLOCK -U $SOCK $ time qemu-img convert -n -f raw -O raw /var/tmp/fedora-27.img nbd:unix:$SOCK real 0m13.491s user 0m0.129s sys 0m0.612s --- plugins/file/file.c | 1 + 1 file changed, 1 insertion(+) diff --git a/plugins/file/file.c b/plugins/file/file.c index 4210adb..fb20622 100644 --- a/plugins/file/file.c +++ b/plugins/file/file.c @@ -41,6 +41,7 @@ #include <sys/types.h> #include <sys/stat.h> #include <errno.h> +#include <linux/falloc.h> /* For FALLOC_FL_* on RHEL, glibc < 2.18 */ #include <nbdkit-plugin.h> -- 2.17.1