NFS on 10G interface terribly slow

Rick Macklem rmacklem at
Fri Jun 26 23:58:45 UTC 2015

Gerrit Kuhn wrote:
> On Thu, 25 Jun 2015 20:49:11 -0400 (EDT) Rick Macklem
> <rmacklem at> wrote about Re: NFS on 10G interface terribly slow:
> RM> Recent commits to stable/10 (not in 10.1) done by Alexander Motin
> RM> (mav@) might help w.r.t. write performance (it avoids large writes
> RM> doing synchronous writes when the wcommitsize is exceeded). If you can
> RM> try stable/10, that might be worth it.
> Ok, I'll schedule an update then, I guess. OTOH, Scott reported that a
> similar setup is working fine for him with 10.0 and 10.1, so there is
> probably not much to gain. I'll try anyway...
> RM> Otherwise, the main mount option you can try is "wcommitsize", which
> RM> you probably want to make larger.
> Hm, which size would you recommend? I cannot find anything about this
> setting, not even what the default value would be. Is this reflected in
> some sysctl, or how can I find out what the actual value is?
The default (auto tuned) value is reported by "nfsstat -m".
It can be set with a mount option (should be something in "man mount_nfs").
If you are doing a test with 1 megabyte writes, I'd set it to at least
1 megabyte. (Basically, writing will be slower for write(2) syscalls that
are larger than wcommitsize. After mav@'s patch, the difference isn't nearly
as noticable. His other commit makes the auto tuned value more reasonable).

If you set it large enough with the "wcommitsize=<N>" mount option, you
don't need the updates stable/10.


> cu
>   Gerrit

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