Re: Increase FreeBSD NFSv4 nfsd buffer size? Re: Increase default NFSv4 server size "max_block_size" to 4MB

On Wed, May 13, 2026 at 4:38 PM Rick Macklem <[email protected]> wrote:
>
> On Wed, May 13, 2026 at 1:37 AM Cedric Blancher
> <[email protected]> wrote:
> >
> > On Wed, 13 May 2026 at 01:02, Rick Macklem <[email protected]> wrote:
> > >
> > > On Tue, May 12, 2026 at 6:18 AM Cedric Blancher
> > > <[email protected]> wrote:
> > > >
> > > > On Tue, 17 Mar 2026 at 00:35, Rick Macklem <[email protected]> wrote:
> > > > >
> > > > > On Mon, Mar 16, 2026 at 5:41 AM Chuck Lever <[email protected]> wrote:
> > > > > >
> > > > > >
> > > > > > On Mon, Mar 16, 2026, at 3:51 AM, Cedric Blancher wrote:
> > > > > > > As debated a while ago, can the default NFSv4 server size for
> > > > > > > "max_block_size" be increased to 4MB, please?
> > > > > >
> > > > > > There is an administrative setting to raise this limit for
> > > > > > recent versions of the kernel. Can you report your experience
> > > > > > when you raise the limit? Hiccups, performance issues, etc? I
> > > > > > would kind of like this exercise to be data-driven.
> > > > > >
> > > > > > What is still unknown to me is which NFS client implementations
> > > > > > can support 4MB or 8MB. Without client support, an increase in
> > > > > > the default in NFSD doesn't mean anything. Rick, Anna, Roland?
> > > > > Although it has not seen much testing, it is possible to do a > 1Mbyte NFSv4
> > > > > mount in FreeBSD.
> > > > > For a 2Mbyte mount, (the only size > 1Mbyte I've tried) the settings would be..
> > > > > In /boot/loader.conf
> > > > > kern.maxphys=2097152
> > > > > vfs.maxbcachebuf=2097152
> > > > >
> > > > > and in /etc/sysctl.conf
> > > > > kern.ipc.maxsockbuf=9455616
> > > > >
> > > > > Then a mount will use 2Mbytes if the server supports it.
> > > >
> > > > How can I verify that the FreeBSD NFSv4 nfsd now uses 2M for NFS buffers?
> > > The default is 128K. (You can see what it is via "sysctl vfs.nfsd.srvmaxio".)
> > >
> > > With "main",
> >
> > Which FreeBSD version is "main" exactly, i.e. which 16.0 snapshot
> > date? Is this supported for 15.0?
The commit just hit "main" as b92b9da33006. So any snapshot after
to-day should have it. (It will be MFC'd to stable/15 in 2 weeks and will
be in 15.2.)

To enable it, put this line in /etc/rc.conf:
nfs_server_maxio=4194304
and this line in /etc/sysctl.conf:
kern.ipc.maxsockbuf=18892800 (or larger)

rick

> >
> > > you can increase that up to 4M by putting
> > > nfs_server_maxio=N (where N can be up to 4194304 for "main" and 1048576 for 15)
> >
> > What line do I have to put into /etc/sysctl.conf and /etc/rc.conf?
> Oops, my bad. It isn't committed yet. I forgot that I have to convince the
> FreeBSD collective to increase UIO_MAXIOV or convince the ZFS folk
> to not check for that limit in ZFS.
>
> I will email when it gets into "main".
>
> It can currently be set to 1Mbyte via
> nfs_server_maxio=1048576
> in /etc/rc.conf and
> kern.ipc.maxsockbuf=5242880  (the exact minimum setting is printed
>                                                     out if you boot
> without this set large
>                                                      enough in
> /etc/sysctl.conf.)
>                                                    --> it must be at
> least a little more than
>                                                         4 *
> nfs_server_maxio so that 4 reads
>                                                         or writes fit
> in the socket's buffers.
>
> rick
>
> >
> > > - adding an entry in /etc/sysctl.conf for a larger value for kern.ipc.maxsockbuf
> > >   (It will tell you the recommended minimum setting if you boot after
> > >    putting nfs_server_maxio=N in /etc/rc.conf.)
> >
> > How can I verify that the settings work, on the NFS server side?
> Look in a wireshark packet trace for maxread and maxwrite in a
> GETATTR reply.
>
> > Can
> > we programmatically probe which values are supported?
> The sysctl is vfs.nfsd.srvmaxio and, when you try and set this
> to too large a value, it will fail with EINVAL.
> (You can do this from C via the sysctl(3) library call. See "man 3 sysctl".)
>
> rick
>
> >
> > Ced
> > --
> > Cedric Blancher <[email protected]>
> > [https://plus.google.com/u/0/+CedricBlancher/]
> > Institute Pasteur
> >