Re: [Libguestfs] [PATCH libnbd] PROPOSAL Add nbdcp (NBD copying) tool.
On Wed, Jan 22, 2020 at 02:40:37PM +0000, Richard W.M. Jones wrote:
This is a proposal for an NBD to/from file copying tool (not
actually
written). Obviously this would duplicate functionality which is
already available in qemu-img convert.
The reasons for writing this tool would be:
- to produce a tool which is very focused on the specific needs of
virt-v2v and similar migration scenarios
It isn't going to need to use qcow2 ever ?
- to have a small tool with minimal dependencies
If you only care about qemu-img with raw files, then QEMU's
dependancies are minimal - little more than glib I expect.
- fix some of the obvious problems with qemu-img convert like how
it
handles devices which are already zeroed, and preallocation
- enable optimizations which make sense for NBD but which might not
be applicable to qemu (eg. multi-conn, freely writing out of order
from multiple threads).
I'd expect multi-conn to be in-scope for QEMU. We have multi-conn
live migration data streams for memory copy, and for the same
reasons as this, I'd expect QEMU will need multi-conn live
migration data streams for disk copy which uses NBD.
I'm not sure what you mean by writing out of order from many
threads, but to support multi-conn, it would have to support
concurrent writers.
Alternative is of course to not write a new tool and instead try to
fix all this stuff in qemu-img itself.
That would be my first preference - or at least include the QEMU
maintainers on this message to determine whether addressing it in QEMU
is practical in a reasonable time frame.
Regards,
Daniel
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