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Re: [PATCH v4 17/18] qapi: backup: add immutable-source parameter


From: Hanna Reitz
Subject: Re: [PATCH v4 17/18] qapi: backup: add immutable-source parameter
Date: Thu, 24 Feb 2022 14:05:16 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0

On 16.02.22 20:46, Vladimir Sementsov-Ogievskiy wrote:
We are on the way to implement internal-backup with fleecing scheme,
which includes backup job copying from fleecing block driver node
(which is target of copy-before-write filter) to final target of
backup. This job doesn't need own filter, as fleecing block driver node
is a kind of snapshot, it's immutable from reader point of view.

Let's add a parameter for backup to not insert filter but instead
unshare writes on source. This way backup job becomes a simple copying
process.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
  qapi/block-core.json      | 11 ++++++-
  include/block/block_int.h |  1 +
  block/backup.c            | 61 +++++++++++++++++++++++++++++++++++----
  block/replication.c       |  2 +-
  blockdev.c                |  1 +
  5 files changed, 69 insertions(+), 7 deletions(-)

I’m not really technically opposed to this, but I wonder what the actual benefit of this is.  It sounds like the only benefit is that we don’t need a filter driver, but what’s the problem with such a filter driver?

(And if we just want to copy data off of a immutable node, I personally would go for the mirror job instead, but it isn’t like I could give good technical reasons for that personal bias.)




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