[SSI] Re: Some issues with GFS

[Brian J. W. <Bri...@co...>]

Christoph Hellwig wrote:
> GNBD currently is rather orphaned.  As I've gotten quite a few bug reports
> lately and lots of people seem to use it I think I will look into this
> issues rather than declaring it obsolete.
> 

Cool. It's useful for testing when you don't have a lot of (or any)
shared storage hardware.


> This is a limitiation in the loopback block driver, not GFS.
> Loop assumes the backing files uses the generic_file_ file operations
> and it can blindly call the address_space operations.
> OpenGFS actually uses these file operations, but requires additional
> locking around them.  One could hack loop.c to do this locking when
> the backing file is on OpenGFS, but I don't think that it is worth
> the effort.
> 

That makes sense.

Loopback mount is used by mkinitrd. I'd like it to work when a SSI
cluster is booted with a shared GFS root. Maybe I'll hack the
mkinitrd.ssi script to build the initrd on the CLMS master's internal
boot disk.


> When you add -DGFS_SHARED_WRITEABLE_MMAP to the CFLAGS for the filesystem
> module you in theory get shared writeable mmaps.  I say in theory because
> the first such mmap owns a glock on the whole file..
> 
> Real shared writeable mmaps are one of the high priority features after
> 0.1, but I'm still not completly sure about the lock design we need for it.
> 

I saw that, but was wary of it. The comment, "gfs_file_mmap - We don't
support shared writable mappings right now", scared me off. ;)


> >   - Lengthy hangs sometimes occur while accessing a GFS partition via
> >     GNBD. I can't remember if I've noticed similar hangs with directly
> >     connected GFS partitions.
> 
> Gnbd again, hmm.
> Can you give me kdb or crash traces for the hanging processes?
> 

Sure. Next time it happens.

-- 
Brian Watson                | "Now I don't know, but I been told it's
Linux Kernel Developer      |  hard to run with the weight of gold,
Open SSI Clustering Project |  Other hand I heard it said, it's
Compaq Computer Corp        |  just as hard with the weight of lead."
Los Angeles, CA             |     -Robert Hunter, 1970

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