[Gluster-devel] regressions due to 64-bit ext4 directory cookies

[Joe Julian]

As an observer who can't really contribute to this issue from a coding 
standpoint, I can only look at the philosophies that come from Linus 
with regard to kernel vs user. From my understanding of Linus' viewpoint 
on the subject:

"If a change results in user programs breaking, it's a bug in the
kernel. We never EVER blame the user programs."

Seems to me, then, that what should have happened is an option should have been made available to allow the 64 bit cookie as an option.

On 02/13/2013 02:20 PM, Theodore Ts'o wrote:
> On Wed, Feb 13, 2013 at 01:21:06PM -0800, Anand Avati wrote:
>> NFS uses the term cookies, while man pages of readdir/seekdir/telldir calls
>> them "offsets".
> Unfortunately, telldir and seekdir are part of the "unspeakable Unix
> design horrors" which has been with us for 25+ years.  To quote from
> the rationale section from the Single Unix Specification v3 (there is
> similar language in the Posix spec).
>
>      The original standard developers perceived that there were
>      restrictions on the use of the seekdir() and telldir() functions
>      related to implementation details, and for that reason these
>      functions need not be supported on all POSIX-conforming
>      systems. They are required on implementations supporting the XSI
>      extension.
>
>      One of the perceived problems of implementation is that returning
>      to a given point in a directory is quite difficult to describe
>      formally, in spite of its intuitive appeal, when systems that use
>      B-trees, hashing functions, or other similar mechanisms to order
>      their directories are considered. The definition of seekdir() and
>      telldir() does not specify whether, when using these interfaces, a
>      given directory entry will be seen at all, or more than once.
>
>      On systems not supporting these functions, their capability can
>      sometimes be accomplished by saving a filename found by readdir()
>      and later using rewinddir() and a loop on readdir() to relocate
>      the position from which the filename was saved.
>
>
> Telldir() and seekdir() are basically implementation horrors for any
> file system that is using anything other than a simple array of
> directory entries ala the V7 Unix file system or the BSD FFS.  For any
> file system which is using a more advanced data structure, like
> b-trees hash trees, etc, there **can't** possibly be a "offset" into a
> readdir stream.  This is why ext3/ext4 uses a telldir cookie, and it's
> why the NFS specifications refer to it as a cookie.  If you are using
> a modern file system, it can't possibly be an offset.
>
>> You can always say "this is your fault" for interpreting the man pages
>> differently and punish us by leaving things as they are (and unfortunately
>> a big chunk of users who want both ext4 and gluster jeapordized). Or you
>> can be kind, generous and be considerate to the legacy apps and users (of
>> which gluster is only a subset) and only provide a mount option to control
>> the large d_off behavior.
> The problem is that we made this change to fix real problems that take
> place when you have hash collisions.  And if you are using a 31-bit
> cookie, the birthday paradox means that by the time you have a
> directory with 2**16 entries, the chances of hash collisions are very
> real.  This could result in NFS readdir getting stuck in loops where
> it constantly gets the file "foo.c", and then when it passes the
> 31-bit cookie for "bar.c", since there is a hash collision, it gets
> "foo.c" again, and the readdir never terminates.
>
> So the problem is that you are effectively asking me to penalize
> well-behaved programs that don't try to steel bits from the top of the
> telldir cookie, just for the benefit of gluster.
>
> What if we have an ioctl or a process personality flag where a broken
> application can tell the file system "I'm broken, please give me a
> degraded telldir/seekdir cookie"?  That way we don't penalize programs
> that are doing the right thing, while providing some accomodation for
> programs who are abusing the telldir cookie.
>
> 					- Ted
>
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