On 2017-05-09, to...@tuxteam.de <to...@tuxteam.de> wrote: > > On Mon, May 08, 2017 at 04:03:54PM -0700, Sergei G wrote: >> And if you think application consistency is nothing to worry about know >> that Git on Ext4 has failed to recover. >> >> My current conclusion: >> >> 1. If I want application consistency I have to take down any application >> that has an internal database or backup those applications prior to FS >> backup. > > That depends. There are applications designed with (on-disk) consistency > in mind (I mention PostgreSQL, I start sounding like a broken record). > > But whether you have to "take down" the application or just "alert" it > for the time the snapshot takes, without its cooperation you either risk > inconsistent state or bad performance (there are cases where apps chose > number two, because they aren't designed for high throughput anyway). > > The gist is: > > (a) file system magic can't "do" it alone > (b) most of the time, a naive "snapshot" with rsync will do > (b') you alone can decide, based on your apps & requirements, > whether the above "most of the time" works for you > > With snapshotting file systems (be it native or assisted) you get just > faster snapshots, i.e. you mitigate the problem. If you want to go all > the way, you have to get the app's cooperation, either voluntary (it's > designed for that) or forced (shutdown)
Because someone mentioned Btrfs and its backup "magic," I'm looking at the wiki, which says (amongst other things): https://btrfs.wiki.kernel.org/index.php/Incremental_Backup Instant, Atomic COW Snapshots Since the snapshots are atomic, when a snapshot is restored it appears to applications as if a power loss had occurred (and the filesystem has gone back to an earlier state). Thus it is possible to backup databases without stopping them beforehand. I don't know what an atomic cow is (maybe some poor bovine living near Chernobyl) but it sounds interesting. I wonder if Btrfs has any notable drawbacks as a file system. BTW, I learned (by way of your least favorite search engine) that the inventor of the Monoyer chart (Ferdinand Monoyer) embedded his name (and occupation--DM, docteur en médecine) in the chart (beginning on the penultimate line, the last and first letters read vertically upwards). These French are clever devils. > There's no magic. > > cheers > - -- tomás > "all generalizations suck" > > -- "It might be a vision--of a shell, of a wheelbarrow, of a fairy kingdom on the far side of the hedge; or it might be the glory of speed; no one knew." --Mrs. Ramsay, speculating on why her little daughter might be dashing about, in "To the Lighthouse," by Virginia Woolf.
