Re: [Qemu-devel] [Qemu-block] [PATCH 0/5] Discrad blocks during block-stream operation

[Peter Krempa]

On Thu, Nov 22, 2018 at 21:48:02 +0300, Andrey Shinkevich wrote:
> Hello everyone!
> 
> The given feature discards blocks with copy-on-read operation while the
> streaming process runs. Adding the 'discard' argument to the QMP block-stream
> command allows dropping a block in the backing chain after it has been copied
> to the active layer. That will elude the block duplication in the intermediate
> backing file. It saves the disk space while external snapshots are being
> merged.

So you specifically want to merge the snapshot by pulling rather than
commiting? Do you have any specific reasons for that? I'm curious
because I plan to finally finish external snapshots in libvirt.

Allowing to pull into intermediate layers will be (or is?) very welcome
by libvirt since I plan to do external snapshot deletion/merging and
that will be greatly simplified by pulling.

On the other hand libvirt will not be able to always use 'discard' as
libvirt's API allows creating alternate histories for a VM and in such
case when merging a snapshot at a branching point we'll need to pull it
into multiple images. The 'discard' optimization can then be used only
with the last branch.

Libvirt's reasons for using 'block-stream' are mostly as it corresponds
to the operations necessary for not messing up the relationship between
the snapshot and which files on disk belong to it.

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