JLS2009: A Btrfs update

Just a week ago I asked dm-devel about the narrow case of dm-snapshot optimally responding to discard-requests. Simply discarding unneeded exception chunks to optimize the amount of cow storage used. I have yet to get any buy-in. One response I did get was about snapshot-origin, which in the article is the lvm snapshot 10minute vs 2 seconds example. For snapshot-origin and other raids, it does as you say require additional bitmap/mask storage and complexity. But for just dm-snapshots, I think it is really simple and beneficial to take advantage of discard requests (unless of course there is some preclusive aspect I don't grok yet). The big benefit I'm interested in is the Fedora/CentOS persitent LiveUSB, utilizing dm-snapshot instead of the more typical unionfs. Currently dm-snapshot suffers in file create/delete problem of cow blocks being used for blocks the filesystem doesn't care about any longer. But if discard-requests can fix that...

https://www.redhat.com/archives/dm-devel/2009-October/msg...