Re: BIG files & file systems

[Hans Reiser <reiser@namesys.com>]

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