Or perhaps simply gzip of the / partition before upgrading. If insuperable problems, zap new / and restore old known working one?

Assuming data/sites are on different partitions.

While initially conceptually easier to grasp, that is far inferior to using a snapshot.

Here's a short list of ways in which that may cause you problems:

1) gzip of a path is not point in time, synced files may no longer be in sync since they were backed up at slightly different times (e.g. I hope you didn't expect database consistency to actually mean anything).

2) gzip of a path will take a while, because it has to actually function on every file (a snapshot is generally copy on write, meaning it's "free" (not quite) for every file until it's changed. Throw away the snapshot before a change and there's no need to copy the file.

3) gzip will take more room (see #2)

Excellent reply. The implications of a significant number of transactions per minute had escaped me.