Libguestfs - Feb 2022 - [PATCH v2v] vddk: Use blocksize filter to split very large requests

Predictably this makes no difference to the current code since nbdcopy
is using 256K requests.  I will do some real testing when Nir pushes
his change here:

https://listman.redhat.com/archives/libguestfs/2022-February/msg00188.html

Nir - do you want to actually increase that back to 8M?  In theory
(untested) it should be safe provided it's combined with something
like this patch.

Rich.

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

On Tue, Feb 15, 2022 at 01:43:01PM +0000, Richard W.M. Jones
wrote:
> Predictably this makes no difference to the current code since nbdcopy
> is using 256K requests.  I will do some real testing when Nir pushes
> his change here:
> 
> https://listman.redhat.com/archives/libguestfs/2022-February/msg00188.html
Some test results can be found in this bug, unfortunately in private
comments.  I'll reproduce relevant excerpts below.  As you can see
this change + 8M block size approximately doubles the copying
performance and brings it roughly in line with virt-v2v 1.45.3.

https://bugzilla.redhat.com/show_bug.cgi?id=2039255


<quote>
>From mxie Comment 31:
1. Convert a guests from ESXi7.0 to rhv(rhv-upload) via vddk7.0.2 with
rhv-direct=true to compare the performance between virt-v2v-1.45.98-1 and
virt-v2v-1.45.3-3

v2v_version    esx7.0-vddk7.0.2-rhv-upload-rhv-direct_true

1.45.98-1      Convert guest: 2m19s              
               Copying disk:  8m18s        
                               
1.45.3-3       Convert guest: 2m5s        
               Copying disk:  4m28s        

>From Richard W.M. Jones Comment 32:
virt-v2v-1.45.98-1.el9.bz2039255.3:

 - Increases nbdcopy --requests to 1024

 - Increases nbdcopy --request-size to 8M

 - Introduces the blocksize filter to prevent VMware problems related to above
changes:
   https://listman.redhat.com/archives/libguestfs/2022-February/msg00211.html


>From mxie Comment 33:
I think the performance has been improved by virt-v2v-1.45.98-1.el9.bz2039255.3

Test the bug with virt-v2v-1.45.98-1.el9.bz2039255.3.x86_64

Steps:
1. Convert a guests from ESXi7.0 to rhv(rhv-upload) via vddk7.0.2 with
rhv-direct=true by v2v

v2v_version    esx7.0-vddk7.0.2-rhv-upload-rhv-direct_true

1.45.98-1      Convert guest: 2m15s              
               Copying disk:  4m54s        
                               
1.45.3-3       Convert guest: 2m5s
               Copying disk:  4m28s

</quote>

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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