[Discuss] Cloud-backup solutions for Linux?

[Bill Bogstad]

On Mon, Sep 28, 2015 at 12:35 PM, Rich Pieri <richard.pieri at gmail.com> wrote:
> On 9/28/2015 8:24 AM, Edward Ned Harvey (blu) wrote:
>>
>> something. The guest storage device is a precise snapshot of what the
>> guest storage would have looked like at the instant that the storage
>> snapshot occurred.
>
>
> No, it isn't. Issuing a flush command on the host does not flush guests'
> buffers so what resides on the host's file system will not include what is
> buffered and not committed in the guests. If you make a snapshot of the host
> file system in this state then the guest file systems on the snapshot will
> be inconsistent and possibly unusable depending on the nature of the data.

I think you are both right.   Ed is right because it is a precise
image (snapshot) of an albeit inconsistent filesystem.      While you
are correct, that the filesystem may be inconsistent with
unpredictable results.  While filesystem based snapshotting systems
generally give you snapshots that are file consistent, disk image
based systems typically do not; and neither of them guarantees that
your applications left their data in a consistent state.  The
appropriate way to backup/snapshot/whatever a system depends on what
you care about and exactly how your apps/filesystems/os do things,
etc.   In the old days, people would run dump on filesystems which
were actively being modified and they got away with it (most of the
time...)  What is your risk tolerance?

Bill Bogstad