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 Tested on Fedora 28 and RHEL 7.5. --- v2: - Include <linux/falloc.h> only on Linux v1 was here: https://www.redhat.com/archives/libguestfs/2018-July/msg00083.html plugins/file/file.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/plugins/file/file.c b/plugins/file/file.c index a8a6253..0345115 100644 --- a/plugins/file/file.c +++ b/plugins/file/file.c @@ -42,6 +42,10 @@ #include <sys/stat.h> #include <errno.h> +#if defined(__linux__) +#include <linux/falloc.h> /* For FALLOC_FL_* on RHEL, glibc < 2.18 */ +#endif + #include <nbdkit-plugin.h> #ifndef O_CLOEXEC -- 2.17.1