Re: Cloning / getting a full backup of a BTRFS filesystem

From: Hugo Mills <hugo@carfax.org.uk>
To: Piotr Szymaniak <szarpaj@grubelek.pl>
Cc: "Andrei Borzenkov" <arvidjaar@gmail.com>,
	"Swâmi Petaramesh" <swami@petaramesh.org>,
	"Linux BTRFS Mailinglist" <linux-btrfs@vger.kernel.org>,
	"Michel Bouissou" <michel@bouissou.net>
Subject: Re: Cloning / getting a full backup of a BTRFS filesystem
Date: Wed, 4 Sep 2019 21:20:12 +0100	[thread overview]
Message-ID: <20190904202012.GF2488@savella.carfax.org.uk> (raw)
In-Reply-To: <20190904140444.GH31890@pontus.sran>

On Wed, Sep 04, 2019 at 04:04:44PM +0200, Piotr Szymaniak wrote:
> On Wed, Sep 04, 2019 at 12:03:10PM +0300, Andrei Borzenkov wrote:
> > On Wed, Sep 4, 2019 at 9:16 AM Swâmi Petaramesh <swami@petaramesh.org> wrote:
> > >
> > > Hi list,
> > >
> > > Is there an advised way to completely “clone” a complete BTRFS
> > > filesystem, I mean to get an exact copy of a BTRFS filesystem including
> > > subvolumes (even readonly snapshots) and complete file attributes
> > > including extended attributes, ACLs and so, to another storage pool,
> > > possibly defined with a different RAID geometry or compression ?
> > >
> > 
> > As long as you do not use top level subvolume directly (all data is
> > located in subolumes), send/receive should work.
> > 
> > > The question boils down to getting an exact backup replica of a given
> > > BTRFS filesystem that could be restored to something logically
> > > absolutely identical.
> > >
> > > The usual backup tools have no clue about share extents, snapshots and
> > > the like, and using btrfs send/receive for individual subvols is a real
> > > pain in a BTRFS filesystem that may contain hundreds of snapshots of
> > > different BTRFS subvols plus deduplication etc.
> > >
> > 
> > Shared extents could be challenging. You can provide this information
> > to "btrfs send", but for one, there is no direct visibility into which
> > subvolumes share extents with given subvolume, so no way to build
> > corresponding list for "btrfs send". I do not even know if this
> > information can be obtained without exhaustive search over all
> > extents. Second, btrfs send/receive only allows sharing of full
> > extents which means there is no guarantee of identical structure on
> > receiving side.
> 
> So right now the only answer is: use good old dd?

   If you want an exact copy, including all of the exact UUIDs, yes.

   Be aware of the problems of making block-level copies of btrfs
filesystems, though:
https://btrfs.wiki.kernel.org/index.php/Gotchas#Block-level_copies_of_devices

   Hugo.

-- 
Hugo Mills             | I have a step-ladder. My real ladder left when I was
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