From: Trond Myklebust <trond.myklebust@fys.uio.no>
Subject: Re: Question: When NFS client check dir's permission,
	it	does not check the cache data
Date: Mon, 25 Jun 2007 16:08:19 -0400
Message-ID: <1182802099.6163.33.camel@heimdal.trondhjem.org>
References: <467F8247.9060607@cn.fujitsu.com>
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Cc: nfs@lists.sourceforge.net
To: Wei Yongjun <yjwei@cn.fujitsu.com>
In-Reply-To: <467F8247.9060607@cn.fujitsu.com>
Sender: nfs-bounces@lists.sourceforge.net
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On Mon, 2007-06-25 at 16:52 +0800, Wei Yongjun wrote:
> Hello, everyone
>  When I test NFS client, I found a poblem that, if a dir we do not have 
> permission to write, it will let the server to check permissions when we 
> perform the write op.
> This is comment by source code:
>   * Optimize away all write operations, since the server
>   * will check permissions when we perform the op.
> In my test, the process is like following:
> #touch dir/file
> NFS Server                         NFS Client
>                        <----------      lookup (dir)
> lookup ok      ------------->
>                        <----------      access (dir)   (*1)
> access(read only)   -----------> 
>                        <----------      lookup (file)
> lookup(NOENT)     ------------->
>                        <----------      create (file)  (*2)
> create(NOPERM)     ------------->
> 
> (*1)
> First to check the permissions of that dir ,and will be add to cache data.
> (*2)
> Since the Client had known the permission of the dir, why no used it? 
> Does this effect to NFS client's performance?

You could, but we don't really have good semantics for how to cache
directory information. For files you have close-to-open caching, but for
directories, there is no equivalent. I'd therefore prefer to be
conservative in cases like this, rather than relying on cached
information.

The other question I have is why this is really something worth
optimising for? Are there really applications out there with a workload
that involves lots of attempts to create files in read-only directories?

Trond


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