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SubjectRe: Special Kernel Modification
> Mounting it synchronous will disable caching in the VM.
Who told you that? Synchronous mounting turns off write buffering. Even with
"-o sync" writes will still end up in the page cache, they'll just be
commited immediately. In case you don't believe me, here's a trivial test on
a block device mounted -o sync (ext3):

bodnar42:/mnt# time cat linux-2.4.13.tar.bz2 > /dev/null

real 0m2.837s
user 0m0.040s
sys 0m0.350s
bodnar42:/mnt# time cat linux-2.4.13.tar.bz2 > /dev/null

real 0m0.328s
user 0m0.010s
sys 0m0.240s

Trust me, the factor of 8 performace improvement the second time is not due
to lucky head positioning or anything, that's coming straight out of cache.

> Another possibility is hostfs. You can directly mount a host directory
> inside UML. That can obviously be shared between UMLs, so you again
> eliminate all the duplication.
Er, it will be shared in the host's context, but each VM will still have
their own copy in the page cache. This is no better than a COW'ed block device

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