[Libguestfs] [PATCH v2v] v2v: -it vddk: Don't use nbdkit readahead filter with VDDK (RHBZ#1832805).

[Richard W.M. Jones]

This filter deliberately tries to coalesce reads into larger requests.
Unfortunately VMware has low limits on the size of requests it can
serve to a VDDK client and the larger requests would break with errors
like this:

  nbdkit: vddk[3]: error: [NFC ERROR] NfcFssrvrProcessErrorMsg: received NFC error 5 from server: Failed to allocate the requested 33554456 bytes

We already increase the maximum request size by changing the
configuration on the VMware server, but it's not sufficient for VDDK
with the readahead filter.

As readahead is only an optimization, the simplest solution is to
disable this filter when we're using nbdkit-vddk-plugin.

Thanks: Ming Xie
---
 v2v/nbdkit_sources.ml | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/v2v/nbdkit_sources.ml b/v2v/nbdkit_sources.ml
index 979c3773..e97583a5 100644
--- a/v2v/nbdkit_sources.ml
+++ b/v2v/nbdkit_sources.ml
@@ -97,9 +97,13 @@ let common_create ?bandwidth ?extra_debug ?extra_env plugin_name plugin_args =
   let cmd = Nbdkit.add_filter_if_available cmd "retry" in
 
   (* Adding the readahead filter is always a win for our access
-   * patterns.  However if it doesn't exist don't worry.
+   * patterns.  If it doesn't exist don't worry.  However it
+   * breaks VMware servers (RHBZ#1832805).
    *)
-  let cmd = Nbdkit.add_filter_if_available cmd "readahead" in
+  let cmd =
+    if plugin_name <> "vddk" then
+      Nbdkit.add_filter_if_available cmd "readahead"
+    else cmd in
 
   (* Caching extents speeds up qemu-img, especially its consecutive
    * block_status requests with req_one=1.
-- 
2.18.2