Re: [linux-lvm] how to convert a disk containing a snapshot to a snapshot lv?

From: Stuart D Gathman <stuart@gathman.org>
To: LVM general discussion and development <linux-lvm@redhat.com>
Cc: Zdenek Kabelac <zdenek.kabelac@gmail.com>
Subject: Re: [linux-lvm] how to convert a disk containing a snapshot to a snapshot lv?
Date: Wed, 29 Dec 2021 08:35:15 -0500 (EST)	[thread overview]
Message-ID: <31e6126c-b6ff-42a7-68f5-a3a2782060d4@gathman.org> (raw)
In-Reply-To: <315E5303-D491-4487-BAD4-958420346163@gmail.com>

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On Tue, 28 Dec 2021, Tomas Dalebjörk wrote:

> Yes, it is an incremental backup based of the cow device

I've used such a COW based backup (can't remember the name just now, currently
using DRBD and rsync for incremental mirrors).  The way it worked was to
read and interpret the raw COW device itself and send blocks over the
wire - writing directly to a volume on the remote end.  It did not try
to patch up metadata and use LVM to merge.  You need an intimate
knowledge of COW internals for either approach - BUT the read-only
approach (with plain writes at the other end) is MUCH safer (not going
to trash metadata at either end) and just as efficient on the wire.

I've also used a block device rsync, that read every block on both
sides and compared hashes - but that is obviously a lot more disk io
that using the COW where LVM is already tracking changed blocks.

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