Re: [PLUG] disk image one-liner

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.

The other way to think about this is that xfsdump and only be run on a mounted file system. So wording aside it was designed that way and thus must deal with changes during the dump operation. Otherwise, we would be forced us to mount xfs read-only in order for the dump facility to work.

So the bottom line is you will not get corruption due to partial file system changes during a dump.

Of course to truly be atomic you can freeze a mounted fs, dump and then release it. In practice that is not necessary or even desired if you do dumps during "quiet" times.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
Keith C. Perry, MS E.E.
Owner, DAO Technologies LLC
(O) +1.215.525.4165 x2033
(M) +1.215.432.5167
www.daotechnologies.com

----- Original Message -----
From: "Rich Freeman" <r-plug@thefreemanclan.net>
To: "Philadelphia Linux User's Group Discussion List" <plug@lists.phillylinux.org>

Sent: Wednesday, April 27, 2016 9:58:47 AM
Subject: Re: [PLUG] disk image one-liner

On Wed, Apr 27, 2016 at 9:14 AM, Keith C. Perry
<kperry@daotechnologies.com> wrote:
> XFS dumps are absolutely atomic. When you run the dump operation you're
> getting a consistent representation (snapshot) at that time. Changes can
> safely be made to the file system while the dumps are running and will get
> picked up in the next run of xfsdump.

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.

> There is also the fsarchive program Rich mentioned but I would be wary of
> live dumping file systems that do not natively support it or are not using
> of an LV snapshot. I don't have any experience with it though.

That tool is definitely NOT intended to be used on live filesystems.

You definitely want to understand the filesystems you're backing up
and how to best do it.

--
Rich
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___________________________________________________________________________
Philadelphia Linux Users Group -- http://www.phillylinux.org
Announcements - http://lists.phillylinux.org/mailman/listinfo/plug-announce
General Discussion -- http://lists.phillylinux.org/mailman/listinfo/plug

John Kreno

"Rightful liberty is unobstructed action according to our will within limits drawn around us by the equal rights of others. I do not add 'within the limits of the law' because law is often but the tyrant's will, and always so when it violates the rights of the individual."

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