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List:       linux-nfs
Subject:    Re: [NFS] Redux: poor NFS V3 performance
From:       Jay Weber <jay () lazy ! accessus ! net>
Date:       2000-12-21 11:43:06
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On Wed, 20 Dec 2000, RW Hawkins wrote:

> So the questions I have for the experts are...
> What is the path for Linux NFS server to benefit from a RAID unit with
> battery-backed cache so acks can be faster?
> How unsafe is data on an NFS v2 server using the async flag? Put another way, how
> often does the flush happen?

'man bdflush' will give you a bit of insight on what you can control
regarding buffer flushing.  Also 'cat /proc/sys/vm/bdflush' should list
what the system is currently using.  I haven't done much experimentation
with it myself, so beyond these pointers I'm useless.  I gather minimizing
the flushes per second could also lead to worse performance, etc.

Another trick that some others seem to use is just run your NFS server on
a dependable UPS if you plan to use async and poweroutages are the primary
reason the server may go down.  Server crashes will leave you with lossage
of data though I gather.  Although possibly watchdog mixed with the UPS
may save you some grief in crash scenarios that aren't so severe.

> I seem to recall reading that Network Appliance does write caches under NFS v2 and I
> know it has battery backed cache. Does anyone have experience with how the VA Linux
> NAS is setup, sync or async?

The VA NAS unit uses sync for it's exports by default to insure data
integrity according to SPECS/RFC.  This becomes very important if the
server was downed for some reason, because upon recovery the clients would
otherwise get "Stale Filehandles" on the recovery due to lost data which
was in the server's buffers at time of crash.

This can be easily changed to using 'async' in the /etc/exports as well.

> 
> Thanks again,


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