Re: [Qemu-block] [Qemu-devel] Can I only commit from active image to corresponding range of its backing file by qemu cmd?

[Alberto Garcia]

On Fri 14 Sep 2018 04:19:01 AM CEST, lampahome wrote:

> I figure out one way is to *divide data of device into 1TB chunk* and
> save every chunk into qcow2 image cuz I don't change filesystem, and
> connect with backing chain.

Let's see if I understood what you want:

 1) you have for example a 30TB image that you want to split because the
    filesystem won't allow files larger than 16TB.

 2) you create 3 images of 10TB each (let's call them [A], [B] and [C]).
    [A] contains the range 0-10TB, [B] 10-20TB and [C] 20-30TB.
    They are connected in a backing chain ([A] <- [B] <- [C]).

 3) there's an empty image [D] on top of those, which is where you
    write ([A] <- [B] <- [C] <- [D]).

 4) you want a way to move the data that you write into [D] to the
    appropriate backing image. So if you write 10MB at offset 4TB, and
    4MB at offset 22TB, the first 10MB should go to image [A], and the
    other 4MB should go to image [C].

Is my description correct?

Some possible solutions:

  a) a new driver (or an extension to the Quorum driver) that would
     allow to concatenate several disk images (like RAID-0 but without
     the stripping part).

  b) an extension to the block-commit command in which you would specify
     the range of data that you want to copy.

  c) you could also merge the files offline. That is: first you create a
     new image that would contain [A] plus its modifications, then you
     would replace [A] with this new image. The first part can already
     be done with the existing tools. The replacement part can be done
     with the "blockdev-reopen" command I'm working on.

Berto