This document contains only my personal opinions and calls of judgement, and where any comment is made as to the quality of anybody's work, the comment is an opinion, in my judgement.
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While explaining the particularly demented VM setup to someone I realized that they did not get some important details, because that setup has several layers of dementedness, none of which is fatal by itself, but they all come nicely together:
As to the last point, a large directory tree composed of many small files does not perform that well over NFS either, but that would be a rather better than putting it inside a sparse virtual disk accessed over NFS.
In particular because then that directory tree could be backed on the NFS server itself, saving a colossal number of network accesses and VM load. For a mailstore in particular tree-scanning operations happen rarely as part of the mailstore operation, as they are usually limite to the subtree for one user. But backups and other administrative tasks (such as statistics gathering) do tree-scanning does apply to the whole tree, and being able to do it on the NFS server avoiding any network accesses and the NFS protocol can be a big win.
Well, I have been too busy to finish a large number of draft blog entries over the past few months but today I feel particularly frustrated and I would like to recommend against:
Of course the above initially works well, as there are too few files and users to trigger thrashing, and it is quick and easy even for unwise and unskilled people to setup.
One of the possible consequences if one were to do all this is that the guy inheriting their mess will find that as the disk usage grows and the locality of reference goes down virtual disk usage thrashes triggering timeouts in the virtual machine's disk drivers, and that even backing up its filesystem to move it to a different setup triggers that.