On Tuesday 10 March 2009 11:44, Scott Rohling wrote:
>Along these lines .. does a Linux filesystem on a RO minidisk reflect any
>changes at all if changes are made by a user with RW?
Yes, but you *really* don't want to do that. Your guest with the RO minidisk
will get corrupted data. You see, Linux caches blocks it has read from the
filesystem in memory. So imagine that it reads in a block containing a set
of directory entries and caches that. Now imagine that another guest with RW
access to that filesystem removes that directory. The RO guest won't know
about it: it will still happily use that cached directory block when reading
that directory, which contains references to files that no longer exist.
What happens when it tries to read those files? It reads those blocks from
DASD, which may well have been overwritten by the RW guest with some other
data, because those blocks were freed up when the directory was deleted.
Oops!
Another bad case is if the RO guest has cached some blocks from an executable
file, and the RW guest has overwritten some or all of those blocks, perhaps
with another executable. The RO guest will read in blocks it hadn't cached
and load it as code, but it has been overwritten by something else (other
code, a text file, who knows?). When that process executes whatever is in
the newly-read block: boom! It will seg-fault at best. Or execute some
other code even!
>Is a deactivate/activate necessary? re-LINK? remount? Anyone know the
>minimum necessary action to see changes?
You should just never do this. Do not modify DASD while a Linux guest has it
mounted read-only. There is no way you can know what parts of that DASD are
cached and what is not.
- MacK.
-----
Edmund R. MacKenty
Software Architect
Rocket Software
275 Grove Street · Newton, MA 02466-2272 · USA
Tel: +1.617.614.4321
Email: [email protected]
Web: www.rocketsoftware.com
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