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Re: [Qemu-block] [PATCH v5 4/6] nbd/server: implement dirty bitmap expor

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH v5 4/6] nbd/server: implement dirty bitmap export
Date: Thu, 10 Jan 2019 01:15:42 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.1

On 1/9/19 1:21 PM, Eric Blake wrote:
> Revisiting an older thread:
> On 6/9/18 10:17 AM, Vladimir Sementsov-Ogievskiy wrote:
>> Handle new NBD meta namespace: "qemu", and corresponding queries:
>> "qemu:dirty-bitmap:<export bitmap name>".
>> With new metadata context negotiated, BLOCK_STATUS query will reply
>> with dirty-bitmap data, converted to extents. New public function
>> nbd_export_bitmap selects bitmap to export. For now, only one bitmap
>> may be exported.

>> +    if (bdrv_dirty_bitmap_enabled(bm)) {
>> +        error_setg(errp, "Bitmap '%s' is enabled", bitmap);
>> +        return;
>> +    }
> Why are we restricting things to only export disabled bitmaps?
> I can understand the argument that if the image being exported is
> read-only, then an enabled bitmap _that can be changed_ is probably a
> bad idea (it goes against the notion of the export being read only).
> But if we were to allow a writable access to an image, wouldn't we
> expect that writes be reflected into the bitmap, which means permitting
> an enabled bitmap?

I've now addressed this in my v2 Promote x-nbd-server-add-bitmap to
stable series.

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

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