On Thursday 27 July 2006 16:50, Nix, Robert P. wrote:
>Actually, I don't think you want a shared filesystem r/w to any image while
> it is r/o to several other images. Subtle things change on a read-write
> disk; accessed dates get touched, and things in the directory float. These
> things could make your r/o systems unstable, even if you aren't actively
> changing things on the shared filesystem. Ext2/3 and Reiser are not meant
> to be shared this way, and lots of stuff gets cached in storage...
It's not just the subtle things: changing a filesystem shared r/o by other
guests can crash those other guests. To use the original example, say the
patch you apply on the main system (which has the disk r/w) updates a program
or library the other guests are using sometimes. Those other guests may have
parts of that program or library in their block caches, but the next time
they need to run it other parts get read from disk. Now you've got some
blocks from the new version of the file mixed in with the old (cached)
blocks, and all heck breaks loose when you try to execute it. Maybe your
application dies. Maybe the problem is in a dynamically-loaded kernel module
and the kernel panics.
Regardless, you should never write to a filesystem that is shared by multiple
Linux instances. That's just opening up a big problem window.
- MacK.
-----
Edmund R. MacKenty
Software Architect
Rocket Software, Inc.
Newton, MA USA
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