[pbs-devel] [PATCH proxmox-backup-restore-image 4/4] add workaround kernel patch for vsock panics

[Dietmar Maurer]

Answering myself, after talking with Thomas:

> On 04/26/2021 8:04 PM Dietmar Maurer <dietmar at proxmox.com> wrote:
> 
>  
> Such small buffer sizes can reduce performance. I usually go
> for a minimum of 256K.
> 
> So do we really need this?

This is simply an optimization to reduce space usage. Performance
is still good enough...

> What is a 128 MiB RAM machine???

This is the restore VM ...

> 
> 
> > On 04/26/2021 3:04 PM Stefan Reiter <s.reiter at proxmox.com> wrote:
> > 
> >  
> > Allocation failures for vsock packet buffers occur routinely when
> > downloading more than one stream at the same time, with less then 512
> > MiB of RAM it sometimes even occurs for single downloads.
> > 
> > This appears to fix it in all of my reproducer scenarios, tested with up
> > to 6 downloads at once in a 128 MiB RAM machine.
> > 
> > Signed-off-by: Stefan Reiter <s.reiter at proxmox.com>
> > ---
> >  .../0003-vsock-reduce-packet-size.patch       | 36 +++++++++++++++++++
> >  1 file changed, 36 insertions(+)
> >  create mode 100644 src/patches/kernel/0003-vsock-reduce-packet-size.patch
> > 
> > diff --git a/src/patches/kernel/0003-vsock-reduce-packet-size.patch b/src/patches/kernel/0003-vsock-reduce-packet-size.patch
> > new file mode 100644
> > index 0000000..378da53
> > --- /dev/null
> > +++ b/src/patches/kernel/0003-vsock-reduce-packet-size.patch
> > @@ -0,0 +1,36 @@
> > +From a437d428733881f408b5d42eb75812600083cb75 Mon Sep 17 00:00:00 2001
> > +From: Stefan Reiter <s.reiter at proxmox.com>
> > +Date: Mon, 26 Apr 2021 14:08:36 +0200
> > +Subject: [PATCH] vsock: reduce packet size
> > +
> > +Reduce the maximum packet size to avoid allocation errors in VMs with
> > +very little memory available (since the buffer needs a contiguous
> > +block of memory, which can get rare for 64kB blocks).
> > +
> > +4kB used to be the default, and according to [0] increasing it makes
> > +the difference between ~25Gb/s and ~40Gb/s - certainly a lot faster,
> > +but both within the realm of unreachable for our restore scenario.
> > +
> > +[0] https://stefano-garzarella.github.io/posts/2019-11-08-kvmforum-2019-vsock/