Libguestfs - Jan 2022 - [PATCH v2v 3/3] lib/qemuNBD.ml: Use qemu-nbd --discard=unmap

Final 3 patches needed to fix "make check-slow", specifically the
test-v2v-trim.sh test.  This revealed a nasty bug in our
sparsification if the input disk format is qcow2.

Rich.

Old virt-v2v used the same output format as input format when doing
disk to disk copies.  Modular virt-v2v doesn't do this, it requires
using -of to set the output format to the same as the input format if
that is desired.
---
 tests/test-v2v-trim.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/test-v2v-trim.sh b/tests/test-v2v-trim.sh
index 8eab4792b2..ed6c031df3 100755
--- a/tests/test-v2v-trim.sh
+++ b/tests/test-v2v-trim.sh
@@ -66,7 +66,7 @@ fi
 
 virt-v2v --debug-gc \
          -i disk $d/fedora.qcow2 \
-         -o local -os $d
+         -o local -of qcow2 -os $d
 
 # Test the libvirt XML metadata and a disk was created.
 test -f $d/fedora.xml
-- 
2.32.0

qemu-nbd --shared (-e) flag controls how many clients can connect
concurrently.  0 means unlimited.

We want to allow the sockets to be queried by other processes while
virt-v2v is running and it should be safe to do this.  The default
configuration of qemu-nbd doesn't allow this so add --shared=0.

Note this does not (in current qemu) enable multi-conn because we
aren't using the -r (read-only) flag.
---
 lib/qemuNBD.ml | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/lib/qemuNBD.ml b/lib/qemuNBD.ml
index 12d083aeb7..89c93d7072 100644
--- a/lib/qemuNBD.ml
+++ b/lib/qemuNBD.ml
@@ -90,7 +90,11 @@ let run_unix ?socket { disk; snapshot; format }    (*
Construct the qemu-nbd command line. *)
   let args = ref [] in
   List.push_back_list args
-    ["qemu-nbd"; "-t"; "--pid-file"; pidfile;
"--socket"; socket];
+    ["qemu-nbd";
+     "-t";
+     "--shared=0";
+     "--pid-file"; pidfile;
+     "--socket"; socket];
 
   (* -s adds a protective overlay. *)
   if snapshot then List.push_back args "-s";
-- 
2.32.0

The default for qemu-nbd is to ignore discard requests.  This meant
that for input files in qcow2 format the "Mapping ..."  (ie. fstrim)
step did nothing, all the work was ignored and we copied deleted data
over to the destination.

This was detected by the test-v2v-trim.sh test.

Fixes: commit 255722cbf39afc0b012e2ac00d16fa6ba2f8c21f
---
 lib/qemuNBD.ml | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/qemuNBD.ml b/lib/qemuNBD.ml
index 89c93d7072..54139ce0b4 100644
--- a/lib/qemuNBD.ml
+++ b/lib/qemuNBD.ml
@@ -93,6 +93,7 @@ let run_unix ?socket { disk; snapshot; format }     
["qemu-nbd";
      "-t";
      "--shared=0";
+     "--discard=unmap";
      "--pid-file"; pidfile;
      "--socket"; socket];
 
-- 
2.32.0