I had to completely re-vase a set of virtual machines that were installed by some less thoughtful predecessor into growable VMware virtual disks. Several virtual disks had several hundred thousand extents (measured with filefrag) and a couple had over a million, all mixed up randomly on the real disk. Performance was horrifying (it did not help that there were another two absurd choices in the setup).
I ended up with just the mostly readonly filesystem in the VM disk, and all the writable subtrees mounted via NFS from the host machine, which was much faster. In particular during backups, because I could run the backup program (BackupPC based on RSYNC) on the real machine and remote backup is a very high IO load operation, and running it inside the virtual machines on a virtual disk was much much slower.
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