On Sun 2008-07-13 13:15:43, jim owens wrote: > Pavel Machek wrote: > >>> This means ONLY SOME metadata (or no metadata) is flushed and >>> then all metadata updates are stopped. User/kernel writes >>> to already allocated file pages WILL go to a frozen disk. >> >> That's the difference here. They do write file data, and thus avoid >> mmap()-writes problem. >> >> ...and they _still_ provide auto-thaw. >> Pavel > > One of the hardest things to make people understand is that > stopping file data writes in the filesystem during a freeze > is not just dangerous, it is also __worthless__ unless you > have a complete "user environment freeze" mechanism. Eh? > And unless each independent component has a freeze and they > can all be coordinated, the data in the pipeline is never > stable enough to say "if you stop all writes to disk and > take a snapshot, this is the same as an orderly shutdown, > backup, restore, and startup". If you also freeze data writes, at least snapshot is not worse than _unexpected_ shutdown. And apps should already be ready for unexpected shutdowns (using fsync etc). Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
