Re: NFS performance degradation of local loopback FS.

[Benny Halevy <bhalevy@panasas.com>]

On Jun. 19, 2008, 15:04 +0300, Peter Staubach <staubach@redhat.com> wrote:
> Krishna Kumar2 wrote:
>>>       200 processes:
>>>     
>> By "200 processes", I meant 200 dd's, each reading from /dev/zero and
>> writing to a file on the filesystem. The script "nfs" was run twice, first
>> with
>> a local filesystem and the second time with the same filesystem NFS
>> mounted.
>>
>>   
> 
> Well, you aren't exactly comparing apples to apples.  The NFS
> client does close-to-open semantics, meaning that it writes
> all modified data to the server on close.  The dd commands run
> on the local file system do not.  You might trying using
> something which does an fsync before closing so that you are
> making a closer comparison.

try dd conv=fsync ...

Benny

> 
> All that said, yes, one would expect a slow down.  How much is
> debatable and varies from platform to platform and load to load.
> 
> I would also advise care when running NFS like that.  It is
> subject to deadlock and is not recommended.
> 
>        ps
> 
>> Thanks,
>>
>> - KK
>>
>> linux-nfs-owner@vger.kernel.org wrote on 06/19/2008 12:16:23 PM:
>>
>>   
>>> Hi,
>>>
>>> I am running 2.6.25 kernel on a [4 way, 3.2 x86_64, 4GB] system. The test
>>> is doing I/O on a local ext3 filesystem, and measuring the bandwidth, and
>>> then NFS mounting the filesystem loopback on the same system. I have
>>> configured 64 nfsd's to run. The test script is attached at the bottom.
>>>
>>> My configuration is:
>>>       /dev/some-local-disk  :            /local
>>>       NFS mount /local       :            /nfs
>>>
>>> The result is:
>>>       200 processes:
>>>             /local: 108000 KB/s
>>>             /nfs:     66000 KB/s: Drop of 40%
>>>
>>>       300 processes (KB/s):
>>>             /local: 112000 KB/s
>>>             /nfs:    57000 KB/s: Drop of 50%
>>>
>>> I am not using any tuning, though I have tested with both
>>> sunrpc.tcp_slot_table_entries=16 & 128
>>>
>>> Is this big a drop expected for a loopback NFS mount? Any
>>> feedback/suggestions are very
>>> appreciated.
>>>
>>> Thanks,
>>>
>>> - KK
>>>
>>> (See attached file: nfs)[attachment "nfs" deleted by Krishna
>>>     
>> Kumar2/India/IBM]
>>
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Benny Halevy
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