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Date: Fri, 02 Aug 2002 22:59:04 +0400
From: Hans Reiser <reiser@namesys.com>
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To: Nikita Danilov <Nikita@Namesys.COM>
CC: trond.myklebust@fys.uio.no, Steve Lord <lord@sgi.com>,
       Jan Harkes <jaharkes@cs.cmu.edu>, Alexander Viro <viro@math.psu.edu>,
       "Peter J. Braam" <braam@clusterfs.com>,
       Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: BIG files & file systems
References: <20020731210739.GA15492@ravel.coda.cs.cmu.edu>	<Pine.GSO.4.21.0207311711540.8505-100000@weyl.math.psu.edu>	<20020801035119.GA21769@ravel.coda.cs.cmu.edu>	<1028246981.11223.56.camel@snafu>	<20020802135620.GA29534@ravel.coda.cs.cmu.edu>	<1028297194.30192.25.camel@jen.americas.sgi.com>	<3D4AA0E6.9000904@namesys.com>	<shslm7pclrx.fsf@charged.uio.no>	<3D4ABAE7.6000709@namesys.com>	<15690.49267.930478.333263@laputa.namesys.com>	<15690.50598.11204.868852@charged.uio.no>	<15690.51993.704549.209766@laputa.namesys.com>	<3D4AD00C.8060701@namesys.com> <15690.54248.33518.887768@laputa.namesys.com>
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Nikita Danilov wrote:

>Hans Reiser writes:
> > Nikita Danilov wrote:
> > 
> > >Trond Myklebust writes:
> > > > >>>>> " " == Nikita Danilov <Nikita@Namesys.COM> writes:
> > > > 
> > > >      > But there still is a problem with applications (if any) calling
> > > >      > seekdir/telldir directly...
> > > > 
> > > > Agreed. Note however that the semantics for seekdir/telldir as
> > > > specified by SUSv2 are much weaker than those in our current
> > > > getdents()+lseek().
> > > > 
> > > > >From the Opengroup documentation for seekdir, it states that:
> > > > 
> > > >   On systems that conform to the Single UNIX Specification, Version 2,
> > > >   a subsequent call to readdir() may not be at the desired position if
> > > >   the value of loc was not obtained from an earlier call to telldir(),
> > > >   or if a call to rewinddir() occurred between the call to telldir()
> > > >   and the call to seekdir().
> > > > 
> > > > IOW assigning a unique offset to each and every entry in the directory
> > > > is overkill (unless the user is calling telldir() for all those
> > > > entries).
> > >
> > Forgive the really dumb question, but does this mean we can just store 
> > the last entry returned to readdir in the directory metadata, and 
> > completely ignore the value of loc?
>
>If application is using readdir, then yes: glibc internally maps readdir
>into getdents plus at most one lseek on directory for "adjustment"
>purposes (if I remember correctly, problem is that kernel struct dirent
>has extra field and glibc cannot tell in advance how many of them will
>fit into supplied user buffer).
>
>But if application uses seekdir(3)/telldir(3) directly---then no.
>
It sounds like we could store the loc to key mapping in the file handle 
(a (partial) key is what reiser4 needs to find a directory entry).  I am 
trying to understand if we need to store more than one loc to key 
mapping in the file handle, or if one is enough.  What do people use 
telldir()/seekdir() for in practice?

>
> > 
> > >
> > >Are you implying some kind of ->telldir() file operation that notifies
> > >file-system that user has intention to later restart readdir from the
> > >"current" position and changing glibc to call sys_telldir/sys_seekdir in
> > >stead of lseek? This will allow file-systems like reiser4 that cannot
> > >restart readdir from 32bitsful of data to, at least, allocate something
> > >in kernel on call to ->telldir() and free in ->release().
> > >
> > > > 
> > > > Cheers,
> > > >   Trond
> > >
>
>Nikita.
>
> > >
> > >
> > >  
> > >
> > 
> > 
> > -- 
> > Hans
> > 
> > 
> > 
>
>
>  
>


-- 
Hans



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