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Date:      Sat, 9 Mar 2013 13:00:04 -0500
From:      Garrett Wollman <wollman@freebsd.org>
To:        Rick Macklem <rmacklem@uoguelph.ca>
Cc:        freebsd-fs@freebsd.org, freebsd-net@freebsd.org
Subject:   Re: NFS DRC size
Message-ID:  <20795.30884.330015.123616@hergotha.csail.mit.edu>
In-Reply-To: <1639798917.3728142.1362846452693.JavaMail.root@erie.cs.uoguelph.ca>
References:  <20794.38381.221980.5038@hergotha.csail.mit.edu> <1639798917.3728142.1362846452693.JavaMail.root@erie.cs.uoguelph.ca>

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<<On Sat, 9 Mar 2013 11:27:32 -0500 (EST), Rick Macklem <rmacklem@uoguelph.ca> said:

> around the highwater mark basically indicates this is working. If it wasn't
> throwing away replies where the receipt has been ack'd at the TCP
> level, the cache would grow very large, since they would only be
> discarded after a loonnngg timeout (12hours unless you've changes
> NFSRVCACHE_TCPTIMEOUT in sys/fs/nfs/nfs.h).

That seems unreasonably large.

> Well, the DRC will try to cache replies until the client's TCP layer
> acknowledges receipt of the reply. It is hard to say how many replies
> that is for a given TCP connection, since it is a function of the level
> of concurrently (# of nfsiod threads in the FreeBSD client)
> in the client. I'd guess it's somewhere between 1<->20?

Nearly all our clients are Linux, so it's likely to be whatever Debian
does by default.

> Multiply that by the number of TCP connections from all clients and
> you have about how big the server's DRC will be. (Some clients use
> a single TCP connection for the client whereas others use a separate
> TCP connection for each mount point.)

The Debian client appears to use a single TCP connection for
everything.

So if I want to support 2,000 clients each with 20 requests in flight,
that would suggest that I need a DRC size of 40,000, which my
experience shows is not sufficient with even a much smaller number of
clients.

-GAWollman




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