Re: Fwd: [PATCH 0/2] block/raw: implemented persistent dirty bitmap and ability to dump bitmap content via qapi

[Max Reitz <mreitz@redhat.com>]

Hi,

On 20.03.21 11:01, Patrik Janoušek wrote:
> I'm sorry, but I forgot to add you to the cc, so I'm forwarding the 
> patch to you additionally. I don't want to spam the mailing list 
> unnecessarily.

I think it’s better to still CC the list.  It’s so full of mail, one 
more won’t hurt. :)

(Re-adding qemu-block and qemu-devel, because the discussion belongs on 
the list(s).)

> -------- Forwarded Message --------
> Subject: 	[PATCH 0/2] block/raw: implemented persistent dirty bitmap and 
> ability to dump bitmap content via qapi
> Date: 	Sat, 20 Mar 2021 10:32:33 +0100
> From: 	Patrik Janoušek <pj@patrikjanousek.cz>
> To: 	qemu-devel@nongnu.org
> CC: 	Patrik Janoušek <pj@patrikjanousek.cz>, lmatejka@kiv.zcu.cz
> 
> 
> 
> Currently, QEMU doesn't support persistent dirty bitmaps for raw format
> and also dirty bitmaps are for internal use only, and cannot be accessed
> using third-party applications. These facts are very limiting
> in case someone would like to develop their own backup tool becaouse
> without access to the dirty bitmap it would be possible to implement
> only full backups. And without persistent dirty bitmaps, it wouldn't
> be possible to keep track of changed data after QEMU is restarted. And
> this is exactly what I do as a part of my bachelor thesis. I've
> developed a tool that is able to create incremental backups of drives
> in raw format that are LVM volumes (ability to create snapshot is
> required).

Similarly to what Vladimir has said already, the thing is that 
conceptually I can see no difference between having a raw image with the 
bitmaps stored in some other file, i.e.:

   { "driver": "raw",
     "dirty-bitmaps": [ {
       "filename": "sdc1.bitmap",
       "persistent": true
     } ],
     "file": {
       "driver": "file",
       "filename": "/dev/sdc1"
     } }

And having a qcow2 image with the raw data stored in some other file, i.e.:

   { "driver": "qcow2",
     "file": {
       "driver": "file",
       "filename": "sdc1.metadata"
     },
     "data-file": {
       "driver": "file",
       "filename": "/dev/sdc1"
     } }

(Where sdc1.metadata is a qcow2 file created with
“data-file=/dev/sdc1,data-file-raw=on”.)

To use persistent bitmaps with raw images, you need to add metadata 
(namely, the bitmaps).  Why not store that metadata in a qcow2 file?

Max