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

[lampahome]

Sorry, I need to explain what case I want to do

Todo: I want to *backup a block device into qcow2 format image.*
I met a problem which is the *file size limit of filesystem* ex: Max is
16TB for any file in ext4, but the block device maybe 32TB or more.

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.
*(That's what I said range is different)*
Ex: 1st chunk of device will save into image.000
2nd chunk of device will save into image.001
Nth chunk of device will save into image.(N-1)
...etc

I can see all block device data when I mount image.(N-1) by qemu-nbd cuz
the chunk doesn't overlap and all chunks connect by backing chain.

Now I want to do next thing: *Incremental backup*
When I modify data of 1st chunk, what I thought is to write new 1st chunk
to new image *image.N* and let *imgae.(N-1)* be the backing file of
*image.N* .
That's cuz I want to store the data before modified to roll back anytime.

So now I have two *version of block device(like concept of snapshot)*:
One is image.000 to image.(N-1). I can access the data before modify by
mount image.(N-1) through qemu-nbd
The other one is image.000 to image.N.  I can access the data after modify
by mount image.N through qemu-nbd(cuz the visible 1st chunk are in the
image.N)

Consider about the situation:
000   A - - - - - - - -  <<<<<---  store the 1st chunk of block device
001   - B - - - - - - -
002   - - C - - - - - - (1st state of block device)
003   A' - - - - - - - - <<<<<--- store the 1st chunk of block device, but
data is different
004   - - - D - - - - - (2nd state of block device)
005   - - - - E - - - -  (3rd state of block device)

The original problem is If I want to remove the 2nd state(003 and 004) but
I need to keep the data of 003 and 004.
If I just commit 003, the A' of 003 must be committed into 002 cuz 002 is
the backing file of 003.
I try to figure out some way to let it only commit from 003 into 000.