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Re: [Xen-devel] copy on write memory

To: Kip Macy <kmacy@xxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] copy on write memory
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Thu, 18 Nov 2004 18:55:25 +0000
Cc: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxxx
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> >
> > Is there any documentation on the semantics of DCSS and the hypervisor
> > services that are used? It would be interesting to see if Xen ought to
> > have some similar concept.
> This doesn't really answer your question - but the closest your likely
> to get is at their pubs:
> http://www.vm.ibm.com/pubs/

There's a manual called "How to Improve the Performance of Linux on
z/VM with Execute-In-Place Technology", which is easily googled for.
Turns out that DCSSs are pretty simple things -- just a blob of
physmem that is mapped into phys address space of every Linux
instance, and contains a read-only filesystem image. xip2fs is a
simple read-only fs that serves mmap() requests directly out of the
shared DCSS rather than the private block cache. Looks tedious to
manage because the whole filesystem takes up space in every Linux
memory map all the time, no matter whether a particular file/block is
being used. So you have to be careful to put only frequently accessed
and highly shared files in the filesystem and use twisted symlinks to
link in from their usual location into the mounted xip2fs filesystem. 

 -- Keir

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