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Re: [Qemu-block] [Qemu-devel] [PATCH RESEND v4] drive-mirror: add increm

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH RESEND v4] drive-mirror: add incremental mode
Date: Tue, 12 Mar 2019 08:43:38 -0500
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On 3/12/19 3:19 AM, Vladimir Sementsov-Ogievskiy wrote:

>> So one important point about incremental backups is that you can
>> actually do them incrementally: The bitmap is effectively cleared at the
>> beginning of the backup process (a successor bitmap is installed that is
>> cleared and receives all changes; at the end of the backup, it either
>> replaces the old bitmap (on success) or is merged into it (on failure)).
>>   Therefore, you can do the next incremental backup by using the same bitmap.
> Hmm, I heard in some other thread that Eric finally decided to always start
> backup on copied bitmap in libvirt, so this logic is skipped. Do we need it
> for mirror? Sorry if I'm wrong, Eric, am I?

You are correct that my current libvirt patches do incremental backup
mode with a temporary bitmap (so regardless of whether the temporary
bitmap is cleared on success or left alone is irrelevant, the temporary
bitmap goes away afterwards). But just because libvirt isn't using that
particular feature of qemu's incremental backups does not excuse qemu
from not thinking about other clients that might be impacted by doing
incremental backup with a live bitmap, where qemu management of the
bitmap matters.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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