MediaWiki 1.5 upgrade

Wikimedia's sites were (June 26–28, 2005) upgraded to the new version of our wiki software, MediaWiki 1.5. While each site was upgraded in turn, it was locked to editing but available for reading; all other sites remained online for read and write. This process is now complete.

Below is the FAQ from before the process begun.

FAQs

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How long will the wiki be locked?
  • en.wikipedia.org, by far the largest of our sites, converted in about 22 hours. Other wikis are much smaller and will convert much faster (ranging from a few minutes to a couple of hours).
Why does it take so long to upgrade?
The core table schema has changed significantly. This should make better use of the database's cache and disk I/O, and significantly speed up rename and delete operations on pages with very long edit histories. Unfortunately this does mean upgrading a wiki of size from 1.4 will require some downtime for the schema restructuring, but future storage backend changes should be able to integrate into the new system more easily.
Which wikis have been converted so far?
Which version is being installed exactly?
What's so great about MediaWiki 1.5?