Re: Collapsing I/O size with unstable writes with NFSv3 on 4.15

[bfields@fieldses.org (J. Bruce Fields)]

On Wed, Mar 20, 2019 at 08:47:32AM -0700, Ted Carpenter wrote:
> I'm testing performance of a clustered NFS server using a 4.15 client.
> My test setup involves running IOZone to write a 30GB file to the
> server over NFSv3. The issue is that we occasionally see bad
> performance, especially when unstable writes are enabled on the
> server.

Which server, and how do you enable unstable writes?

> Based on observing the network traffic, we've seen the write I/O size
> during good runs stay around 1MB (which is how we've mounted) but in
> the bad runs we see often see dips in write size (down to even 4KB or
> 8KB) towards the end of the test.
> 
> I'm trying to understand why we see the dip in I/O size from the NFS
> client as it seems to be related to the decrease in performance. Any
> ideas what could cause it? Also, any suggestions on where I should
> look to start learning about the way the Linux NFS client and page
> cache handle unstable writes internally?

I'm not sure what to recommend.  We might also need some more details
about the performance results you're seeing.

--b.