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From 4e0b3de57486613c8f28ef7726df728cccd7624b Mon Sep 17 00:00:00 2001
From: "Richard W.M. Jones" <>
Date: Thu, 28 May 2020 10:59:57 +0100
Subject: [PATCH] v2v: -it vddk: Don't use nbdkit readahead filter with VDDK
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
(cherry picked from commit 1438174488f111fa24420758ba3bf0218dc9ee2a)
v2v/ | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/v2v/ b/v2v/
index 402dfd0e..bfda91a7 100644
--- a/v2v/
+++ b/v2v/
@@ -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.