Re: [PLUG] disk image one-liner

[Rich Freeman <r-plug@thefreemanclan.net>]

On Wed, Apr 27, 2016 at 1:27 PM, Keith C. Perry
<kperry@daotechnologies.com> wrote:
> "Are you saying that if I start a dump that takes an hour to complete,
> then change a bunch of files, the dump will reflect the contents of
> the file as of the moment the dump was started?  That wasn't clear in
> my quick google searching, but it is of course theoretically possible
> with a journaling filesystem."
>
> That is exactly what I am saying  :)
>
> The reason you're not seeing that wording or are even people saying that it is not atomic is because depending on the order of operations, a change to the fs after a dump starts may or may not be included in the dump.  That is fine in practically speaking but if you want to be strict with definitions it isn't.

I'll admit I'm not a computer scientist, but I'm pretty sure the
definition of atomic is that the entire backup looks as if it had
taken no time at all to perform.  So, for it to be atomic any file
must be stored in the state it was in the moment the backup started.
A change to a file after the dump starts must not be included in the
dump, in that case.

Certainly that is the result if you freeze the filesystem and make an
LVM snapshot and then backup the snapshot.  An atomic backup should be
completely internally consistent (well, minus any changes in
applications not committed to disk, which is the advantage of first
unmounting a disk before starting the backup).
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