[Libguestfs] [PATCH v2v] vddk: Use blocksize filter to split very large requests

[Richard W.M. Jones]

We know that VDDK has problems handling very large single requests.
It may cause out of memory errors on the server unless a relatively
obscure server-side configuration change is made.  With modular
virt-v2v we won't necessarily have fine control over who is connecting
to the input socket and what sized requests they may make, eg if we
delegate copying to a third party.

nbdkit-blocksize-filter offers a simple solution.  It will split
requests larger than a certain size, ensuring that whatever NBD
requests come in, we shouldn't cause problems on the VMware server.
I chose 2M as the maximum request size, but this could be fine-tuned
later.

For a longer explanation, see:
https://gitlab.com/nbdkit/nbdkit/-/commit/6f4746af9ac65c779d6afda9459e9f862a26d145
---
 input/nbdkit_vddk.ml | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/input/nbdkit_vddk.ml b/input/nbdkit_vddk.ml
index 40dae9baa7..9609c4423f 100644
--- a/input/nbdkit_vddk.ml
+++ b/input/nbdkit_vddk.ml
@@ -144,6 +144,16 @@ See also the virt-v2v-input-vmware(1) manual.") libNN
    *)
   Nbdkit.add_filter_if_available cmd "cacheextents";
 
+  (* Split very large requests to avoid out of memory errors on the
+   * server.  Since we're using this filter, also add minblock=512
+   * although it will make no difference.
+   *)
+  if Nbdkit.probe_filter "blocksize" then (
+    Nbdkit.add_filter cmd "blocksize";
+    Nbdkit.add_arg cmd "minblock" "512";
+    Nbdkit.add_arg cmd "maxdata" "2M"
+  );
+
   (* IMPORTANT! Add the COW filter.  It must be furthest away
    * except for the multi-conn and rate filters.
    *)
-- 
2.35.1