Re: Regarding client fairness

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Mar 28, 2018 at 10:54:06AM -0400, bfields wrote:
> On Wed, Mar 28, 2018 at 02:04:57PM +0300, daedalus@pingtimeout.net wrote:
> > I came across a rather annoying issue where a single NFS client
> > caused resource starvation for NFS server. The server has several
> > storage pools which are used, in this particular case a single
> > client did fairly large read requests and effectively ate all nfsd
> > threads on the server and during that other clients were getting
> > hardly any I/O through to the other storage pool which was
> > completely idle.
> 
> What version of the kernel are you running on your server?

I'm thinking that if it includes upstream 637600f3ffbf "SUNRPC: Change
TCP socket space reservation" (in upstream 4.8), then you may want to
experiment setting the sunrpc.svc_rpc_per_connection_limit module
parameter added in ff3ac5c3dc23 "SUNRPC: Add a server side
per-connection limit".

You probably want to experiment with values greater than 0 (the default,
no limit) and the number of server threads.

--b.

> 
> --b.
> 
> > 
> > I then proceeded to make a simple testcase and noticed that reading
> > a file with large blocksize causes NFS server to read using multiple
> > threads, effectively consuming all nfsd threads on the server and
> > causing starvation to other clients regardless of the share/backing
> > disk they were accessing.
> > 
> > In my testcase a simple (ridiculous) dd was able to effectively
> > reserve the entire NFS server for itself:
> > 
> > # dd if=fgsfds bs=1000M count=10000 iflag=direct
> > 
> > Also several similar dd runs with blocksize of 100M caused the same
> > effect. During those dd-runs the server was responding at a very
> > slow rate to any other requests by other clients (or to other NFS
> > shares on different disks on the server).
> > 
> > My question here is that are there any methods to ensure client
> > fairness with Linux NFS and/or are there some best common practices
> > to ensure something like that. I think it would be pretty awesome if
> > clients had some kind of limit/fairness that would be scoped like
> > {client, share-on-server} so client which accesses a single share on
> > a server (with large read IO requests) would not effectively cause
> > denial of service for the entire NFS server but rather only to the
> > share it is accessing and at same time other clients accessing
> > different/same share would get fair amount of access to the data.
> > --
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