Re: [PATCH v2 3/4] qapi: blockdev-backup: add discard-source parameter

[Vladimir Sementsov-Ogievskiy]

On 19.01.24 17:46, Fiona Ebner wrote:
> Am 17.01.24 um 17:07 schrieb Vladimir Sementsov-Ogievskiy:
> > Add a parameter that enables discard-after-copy. That is mostly useful
> > in "push backup with fleecing" scheme, when source is snapshot-access
> > format driver node, based on copy-before-write filter snapshot-access
> > API:
> >
> > [guest]      [snapshot-access] ~~ blockdev-backup ~~> [backup target]
> >     |            |
> >     | root       | file
> >     v            v
> > [copy-before-write]
> >     |             |
> >     | file        | target
> >     v             v
> > [active disk]   [temp.img]
> >
> > In this case discard-after-copy does two things:
> >
> >   - discard data in temp.img to save disk space
> >   - avoid further copy-before-write operation in discarded area
> >
> > Note that we have to declare WRITE permission on source in
> > copy-before-write filter, for discard to work.
> >
> > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
>
> Unfortunately, setting BLK_PERM_WRITE unconditionally breaks
> blockdev-backup for a read-only node (even when not using discard-source):

Ohh, right.

So, that's the place when we have to somehow pass through discrard-souce option 
to CBW filter, so that it know that WRITE permission is needed. Will try in v3. 
Thanks again for testing!

> > #!/bin/bash
> > ./qemu-img create /tmp/disk.raw -f raw 1G
> > ./qemu-img create /tmp/backup.raw -f raw 1G
> > ./qemu-system-x86_64 --qmp stdio \
> > --blockdev 
> > raw,node-name=node0,file.driver=file,file.filename=/tmp/disk.raw,read-only=true 
> > \
> > --blockdev raw,node-name=node1,file.driver=file,file.filename=/tmp/backup.raw \
> > <<EOF
> > {"execute": "qmp_capabilities"}
> > {"execute": "blockdev-backup", "arguments": { "device": "node0", "target": "node1", "sync": "full", 
> > "job-id": "backup0" } }
> > EOF
>
> The above works before applying this patch, but leads to an error
> afterwards:
>
> > {"error": {"class": "GenericError", "desc": "Block node is read-only"}}
>
> Best Regards,
> Fiona

--
Best regards,
Vladimir