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Re: [Xen-devel] double or triple access to ext2, ext3 or other partitions



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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