Re: reiser4 plugins

[David Masover]

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Kyle Moffett wrote:
> On Jun 27, 2005, at 17:19:21, David Masover wrote:
> 
>>> Precisely.  Come back when you have a good sturdy set of  arguments. 
>>> See
>>> the FUSE project (Also discussed in this thread), for better ideas 
>>> on  how
>>> to add strange filesystem semantics.
>>
>>
>> If I didn't care about performance.  I will read up on how FUSE works,
>> though, to see if there's anything in the _kernel_ code that would be
>> useful.
> 
> 
> A number of projects do their performance critical things in userspace,
> including real-time audio and video editing.  The kernel is *NOT* for
> performance-critical things, unless they cannot be done efficiently in
> userspace.

I don't think transparent compress-on-flush can be done efficiently in
userspace.

> It is for _security_critical_ and _stability_critical_  things,
> and even then, some of those are pushed to userspace as well.

There's more to it than that.  It is also for certain interfaces that
makes more sense in kernel space, or are there for historical reasons.
For instance, a filesystem could be done in userspace, with a userspace
library, but even FUSE uses the kernel for its interface, rather than
(say) glibc.

>>> NOTE:  Even the FUSE project,  which
>>> is in _userspace_ (as opposed to the massive kernelspace mess of  
>>> reiser4),
>>> is taking a lot of flak for changing UNIX semantics, so I see no  reason
>>> why Reiser4 should be special.
>>
>>
>> Right.  Kernel people hate change.  Got it.
>>
>> No, seriously, I have to respect the history involved, which is why  I'm
>> not pretending to know more than I do.
> 
> There is a good reason to hate change (at least without thorough  evidence
> to back it up).  Even small changes can break things in subtle ways.   Big
> changes can tend to wreak kernel-global (and even userspace-global)  havoc.

Right on all points.  Just remember that some change is good.  Why do we
have ALSA now?  Everything a user can do with ALSA, they can do with
OSS, AFAIK.

>>> Ok, good.  That's one of the first issues.  A _lot_ of applications
>>> would get themselves thoroughly confused at that '...' interface, not
>>> to  mention a lot of sysadmins :-D.
>>>
>>
>> Not as much as you think.  Unless a lot of applications can't handle
>> opening or saving files that have '...' in the pathname?
> 
> 
> If those files are as special as you lead me to believe, even a simple
> "tar -cjvf foo.tar.bz2 foo" would break horribly, because it would  tar up
> all sorts of strange special files that shouldn't be included.  Worse,
> when I untar that archive on the filesystem, it will overwrite those 
> files,
> which probably should be overwritten even less than they should be  stored
> in an archive.

Point well taken, although sometimes that's exactly what you want.  You
no longer need the -p flag to Tar that way, although you would want its
opposite, so that you can ignore '...'
> 
>> The sysadmin problem doesn't worry me so much.
> 
> 
> Personally, I know several sysadmins who, never having heard of Reiser4
> but using it anyways, would get scared when all of the '...' directories
> started showing up, thinking that some cracker had gotten a module  loaded
> in their kernel.

I'm not sure if the '...' or '..metas' directory is currently available
in Reiser4, but at one point it was the default, then it got changed to
a mount option which is off by default.

> If you can, please avoid overloading existing  semantics
> in filesystems.  You can either claim that your filesystem is POSIXy,
> (similar to ext3, xfs, etc), or you can claim that it's not  compatible by
> design (AFS, Coda) or you can claim that it's a completely virtualized
> interface (procfs, sysfs, CKRM fs, etc) and doesn't care about POSIX in
> any case.  Reiser4 definitely isn't the third, but neither is it  either of
> the first two, because it would break standard operations.

Reiser4 is attempting to extend POSIX without breaking it too much.

>> Agreed.  I don't know enough about VFS to propose one, though.  I  think
>> others are working on that, finally.
> 
> When the VFS work gets done, this discussion can move on, otherwise, we
> are stuck at an impasse.

Not my department.  It seems that people are agreeing on what needs to
be done, though.

>> Not too much, I hope.
>>
>> Although being able to use different keys on a per-file basis would be
>> nice.  I'm not sure if that would work in the existing system.  Not to
>> mention that keys would also be handlable with '...', if/when it 
>> happens.
> 
> 
> I think the '...' is just a bad idea in general, because it breaks tar
> and such.  An interface like this might be simpler to design and use:
> 
> # mkdir /attr
> # mount -t attrfs attrfs /attr
> 
> /attr/fd/$FD_NUM              == '...' directory for a filedescriptor
> /attr/fs/$DEV_NUM/$INODE_NUM  == '...' directory for an inode
> 
> It would be usable from a shell with a simple "getattrpath" command
> which returns "fs/$DEV_NUM/$INODE_NUM" by stat-ing any given path.

Still pretty annoying, but maybe a good idea, especially if the shell
gets extended to support it.  Not sure I like using inode numbers,
though -- I like the idea of being able to symlink to stuff inside the
meta-file dir.

Actually, I like this.  Give me some time to let it percolate.

Hans, thoughts?  Seems to be namespace fragmentation, but seems usable,
less breakage, and so on.  And should it be /attr or /meta?

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