Xen devel - Nov 2004 - double or triple access to ext2, ext3 or other partitions

dear xen developers,

how feasible would it be, in your opinions, to add in code into the
linux kernel / xen infrastructure, around the ext2 and ext3 filesystem
drivers, that meant, ultimately, that you could have TWO OR MORE
separate linux xen guest OSes mounting the SAME partition?

muhahahahah  oo sorry.

i would envisage this as being implemented somewhere along the following
lines:

1) provide some mutex semaphore system in the xen infrastructure which
one or more linux xen guest OS could "register" with and use

2) at every point in the ext2 and ext3 filesystem drivers where
SMP-driven locking is performed, add in xen-semaphore usage and locking
(as well / instead).

presumably, the issue of locking an ext2, ext3 or other
filesystem already covers the issue of SMP-based locking,
therefore i imagine it wouldn''t be too hard to utilise that
opportunity to put in "alternative" locking.

l.

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> 2) at every point in the ext2 and ext3 filesystem drivers where
> SMP-driven locking is performed, add in xen-semaphore usage and locking
> (as well / instead).
It''s not quite that simple: the ext driver in each domain will be
caching
data related to its filesystem changes in memory.  In an SMP system, both 
CPUs can see this cached data in memory, whereas separate virtual machines 
cannot see what each other are caching.

Mounting the same partition is already possible using a cluster filesystem 
(like GFS).  Cluster filesystems handle the extra locking correctly 
because they''re designed for use with multiple hosts sharing a disk
(e.g.
over a SAN).  They''ll even work with multiple writers.

It is also possible to export any filesystem via NFS and have it mounted 
by multiple domains or to have copy-on-write semantics (e.g. using LVM) so 
that domains never see each others changes (thus circumventing the 
problem).

HTH,
Mark
>
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On Tue, 23 Nov 2004, Luke Kenneth Casson Leighton wrote:
> how feasible would it be, in your opinions, to add in code into the
> linux kernel / xen infrastructure, around the ext2 and ext3 filesystem
> drivers, that meant, ultimately, that you could have TWO OR MORE
> separate linux xen guest OSes mounting the SAME partition?
Try GFS or OCFS2.

-- 
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan


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On Tue, Nov 23, 2004 at 05:30:04PM +0000, Mark Williamson
wrote:
> >2) at every point in the ext2 and ext3 filesystem drivers where
> >SMP-driven locking is performed, add in xen-semaphore usage and locking
> >(as well / instead).
> 
> It''s not quite that simple: the ext driver in each domain will be
caching
> data related to its filesystem changes in memory.  In an SMP system, both 
> CPUs can see this cached data in memory, whereas separate virtual machines 
> cannot see what each other are caching.
 oh drat.
> Mounting the same partition is already possible using a cluster filesystem 
> (like GFS).  Cluster filesystems handle the extra locking correctly 
> because they''re designed for use with multiple hosts sharing a
disk (e.g.
> over a SAN).  They''ll even work with multiple writers.
 
 oooooo :)
> It is also possible to export any filesystem via NFS and have it mounted 
> by multiple domains or to have copy-on-write semantics (e.g. using LVM) so 
> that domains never see each others changes (thus circumventing the 
> problem).
 in that case, the NFS server becomes the single point of failure should
 it become attacked / compromised.

 thank you for your advice about gfs, i will most definitely investigate
 it.

 ta,

 l.



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On Tue, Nov 23, 2004 at 03:14:38PM -0500, Rik van Riel
wrote:
> On Tue, 23 Nov 2004, Luke Kenneth Casson Leighton wrote:
> 
> >how feasible would it be, in your opinions, to add in code into the
> >linux kernel / xen infrastructure, around the ext2 and ext3 filesystem
> >drivers, that meant, ultimately, that you could have TWO OR MORE
> >separate linux xen guest OSes mounting the SAME partition?
> 
> Try GFS or OCFS2.
 thanks!



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