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Re: Live Fille System Backup

On 2017-05-09, tomas@tuxteam.de <tomas@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):


 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.

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