The Conservation Measure is the only sweet note in this discordant symphony we've arranged. It's the only constructive activity in decades of destruction.
The Library Project, also referred to as the Conservation Measure, was the second part of the Forerunners plan to eradicate the Flood from the Milky Way galaxy at the end of the Forerunner-Flood war.
When the first plans were made for the Halo Array, the Librarian was outraged at the Builders' proposal to eradicate the Flood by destroying all of the sentient life in the galaxy. This resulted in a petition to force the Master Builder and the Council to seek other methods, without galactic genocide. Because Lifeworkers were invaluable to medical science, the Master Builder Faber gave in to this demand to prevent a major strike. Because of the great variety in sentient life, there were difficulties with categorizing them into specific categories.
After it became clear that the Halo Arrays were the only thing powerful enough to kill a full-grown Gravemind, the Ur-Didact was urged by his wife to fire the Halo Arrays. However, out of love for his wife, the Didact delayed the activating. However, under pressure of 032 Mendicant Bias' alliance with the Flood, the Librarian destroyed her Keyship on Earth. Stranded on Earth, in the Array's radius of fire, the Didact knew there was no way to save her, without any option left, the Didact fired the Array.
After the Halo Array fired, destroying all sentient life in the Milky Way galaxy, the Ark's Sentinels began to reintroduce all samples of the different races to their own homeworld. This was at a great cost, as all the sentient species were sent back to Tier 7 of the Forerunners "Technological Achievement Tiers", forcing them to start over from a pre-industrial age. This resulted in a troublesome situation, from which some species haven't recovered and became extinct. This time was later referred to the "dark time".
Two of those races later started an alliance which worshiped the Forerunners as gods, something the Forerunners did not expect. Information about the Library Project was stored in the Ark's databases, which was discovered by a surprised 343 Guilty Spark.