From: Peter Staubach <staubach@redhat.com>
Subject: Re: mtime not updated in client cache?
Date: Fri, 09 Dec 2005 14:20:28 -0500
Message-ID: <4399D8FC.1020400@redhat.com>
References: <044B81DE141D7443BCE91E8F44B3C1E201332866@exsvl02.hq.netapp.com>
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Cc: nfs@lists.sourceforge.net
To: "Lever, Charles" <Charles.Lever@netapp.com>
In-Reply-To: <044B81DE141D7443BCE91E8F44B3C1E201332866@exsvl02.hq.netapp.com>
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Lever, Charles wrote:

>
>risks understood.
>
>however, how is this different from ext3?  a close() on an ext3 file
>doesn't even attempt to schedule a flush of the data to disk.  if the
>system crashes or the disk dies or the file system fills up, the data is
>lost, just like in the NFS case.
>  
>

I think that the difference is that a system crash also terminates the
application and there is just more visibility into what happened.

>in addition, mapped dirty data is never flushed on close() on NFS.  what
>happens if a server failure pins dirty mapped pages in the client's
>memory?
>
>  
>

Actually, I think that part of close-to-open processing should be write
out dirty pages on munmap.

>if an application really wants the guarantee that the data is
>permanently stored or have the opportunity to recover if an error
>occurs, it will use O_SYNC or msync() or fflush() before closing the
>file.
>
>  
>

True.  An intermediate position has been to pay attention to system call
return values, including that from close(2), and pass them along to the
user to figure out what to do.

>to make a more philosophical argument, UNIX has always compromised data
>integrity for performance in the common case.  for example, why aren't
>there more file systems around that provide ACID semantics?  because in
>99.9% of cases, ACID semantics are simply not required, and just slow
>everything else down.
>
>i think this is reasonable behavior to enable with either a mount option
>or a sysctl.  we have "nocto," after all.
>
>  
>

I think that "nocto" was invented to be able to optimize situations where it
was known that a specific client was the _only_ client accessing those 
files.
Thus, close-to-open consistency was not required.

I've wondered about the real performance advantage though.  I've never
seen it measured in a believeable way.

>i'm not trying to start an argument.  just wondering aloud.
>  
>

I am happy to have the discussion, but it feels to me like a very dangerous
area to venture into.  Several times, I thought about trying to do this,
but each time stopped because I wasn't comfortable with the possibility
of silent data loss.

    Thanx...

       ps


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