[2/3] qapi: blockdev-backup: add discard-source parameter

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. Alternative is to pass
an option to bdrv_cbw_append(), add some internal open-option for
copy-before-write filter to require WRITE permission only for backup
with discard-source=true. But I'm not sure it worth the complexity.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>
---
 block/backup.c                         |  5 +++--
 block/block-copy.c                     | 10 ++++++++--
 block/copy-before-write.c              |  2 +-
 block/replication.c                    |  4 ++--
 blockdev.c                             |  2 +-
 include/block/block-copy.h             |  2 +-
 include/block/block_int-global-state.h |  2 +-
 qapi/block-core.json                   |  4 ++++
 8 files changed, 21 insertions(+), 10 deletions(-)

Hi Vladimir,

hope I didn't miss a newer version of this series. I'm currently
evaluating fleecing backup for Proxmox downstream, so I pulled in this
series and wanted to let you know about two issues I encountered while
testing. We are still based on 8.1, but if I'm not mistaken, they are
still relevant:

Am 31.03.22 um 21:57 schrieb Vladimir Sementsov-Ogievskiy:
> @@ -575,6 +577,10 @@ static coroutine_fn int block_copy_task_entry(AioTask *task)
>      co_put_to_shres(s->mem, t->req.bytes);
>      block_copy_task_end(t, ret);
>  
> +    if (s->discard_source && ret == 0) {
> +        bdrv_co_pdiscard(s->source, t->req.offset, t->req.bytes);
> +    }
> +
>      return ret;
>  }
>  

If the image size is not aligned to the cluster size, passing
t->req.bytes when calling bdrv_co_pdiscard() can lead to an assertion
failure at the end of the image:

> kvm: ../block/io.c:1982: bdrv_co_write_req_prepare: Assertion `offset + bytes <= bs->total_sectors * BDRV_SECTOR_SIZE || child->perm & BLK_PERM_RESIZE' failed.

block_copy_do_copy() does have a line to clamp down:

> int64_t nbytes = MIN(offset + bytes, s->len) - offset;

If I do the same before calling bdrv_co_pdiscard(), the failure goes away.


For the second one, the following code saw some changes since the series
was sent:

> diff --git a/block/copy-before-write.c b/block/copy-before-write.c
> index 79cf12380e..3e77313a9a 100644
> --- a/block/copy-before-write.c
> +++ b/block/copy-before-write.c
> @@ -319,7 +319,7 @@ static void cbw_child_perm(BlockDriverState *bs, BdrvChild *c,
>          bdrv_default_perms(bs, c, role, reopen_queue,
>                             perm, shared, nperm, nshared);
>  
> -        *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
> +        *nperm = *nperm | BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE;
>          *nshared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
>      }
>  }

It's now:

>         bdrv_default_perms(bs, c, role, reopen_queue,
>                            perm, shared, nperm, nshared);
> 
>         if (!QLIST_EMPTY(&bs->parents)) {
>             if (perm & BLK_PERM_WRITE) {
>                 *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
>             }
>             *nshared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
>         }

So I wasn't sure how to adapt the patch:

- If setting BLK_PERM_WRITE unconditionally, it seems to break usual
drive-backup (with no fleecing set up):

> permissions 'write' are both required by node '#block691' (uses node '#block151' as 'file' child) and unshared by block device 'drive-scsi0' (uses node '#block151' as 'root' child).

- If I only do it within the if block, it doesn't work when I try to set
up fleecing, because bs->parents is empty for me, i.e. when passing the
snapshot-access node to backup_job_create() while the usual cbw for
backup is appended. I should note I'm doing it manually in a custom QMP
command, not in a transaction (which requires the not-yet-merged
blockdev-replace AFAIU).

Not sure if I'm doing something wrong, but maybe what you wrote in the
commit message is necessary after all?

> Alternative is to pass
> an option to bdrv_cbw_append(), add some internal open-option for
> copy-before-write filter to require WRITE permission only for backup
> with discard-source=true. But I'm not sure it worth the complexity.

Best Regards,
Fiona

Hi Fiona!

That was the only version, because it was based on "[PATCH v5 00/45] Transactional block-graph modifying API", as written in commit message.

And "[PATCH v5 00/45] Transactional block-graph modifying API" was not merged, instead I decided to send it part-by-part, some were already merged. Now the current "in-flight" part is "[PATCH v8 0/7] blockdev-replace".

So, probably something from that old big series is still needed for "backup: discard-source parameter" to work. Or, may be everything is prepared now.

I'll look at it closely next week and try to make a v2. Thanks for testing and debugging!

On 11.01.24 18:19, Fiona Ebner wrote:
> Hi Vladimir,
> 
> hope I didn't miss a newer version of this series. I'm currently
> evaluating fleecing backup for Proxmox downstream, so I pulled in this
> series and wanted to let you know about two issues I encountered while
> testing. We are still based on 8.1, but if I'm not mistaken, they are
> still relevant:
> 
> Am 31.03.22 um 21:57 schrieb Vladimir Sementsov-Ogievskiy:
>> @@ -575,6 +577,10 @@ static coroutine_fn int block_copy_task_entry(AioTask *task)
>>       co_put_to_shres(s->mem, t->req.bytes);
>>       block_copy_task_end(t, ret);
>>   
>> +    if (s->discard_source && ret == 0) {
>> +        bdrv_co_pdiscard(s->source, t->req.offset, t->req.bytes);
>> +    }
>> +
>>       return ret;
>>   }
>>   
> 
> If the image size is not aligned to the cluster size, passing
> t->req.bytes when calling bdrv_co_pdiscard() can lead to an assertion
> failure at the end of the image:
> 
>> kvm: ../block/io.c:1982: bdrv_co_write_req_prepare: Assertion `offset + bytes <= bs->total_sectors * BDRV_SECTOR_SIZE || child->perm & BLK_PERM_RESIZE' failed.
> 
> block_copy_do_copy() does have a line to clamp down:
> 
>> int64_t nbytes = MIN(offset + bytes, s->len) - offset;
> 
> If I do the same before calling bdrv_co_pdiscard(), the failure goes away.
> 
> 
> For the second one, the following code saw some changes since the series
> was sent:
> 
>> diff --git a/block/copy-before-write.c b/block/copy-before-write.c
>> index 79cf12380e..3e77313a9a 100644
>> --- a/block/copy-before-write.c
>> +++ b/block/copy-before-write.c
>> @@ -319,7 +319,7 @@ static void cbw_child_perm(BlockDriverState *bs, BdrvChild *c,
>>           bdrv_default_perms(bs, c, role, reopen_queue,
>>                              perm, shared, nperm, nshared);
>>   
>> -        *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
>> +        *nperm = *nperm | BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE;
>>           *nshared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
>>       }
>>   }
> 
> It's now:
> 
>>          bdrv_default_perms(bs, c, role, reopen_queue,
>>                             perm, shared, nperm, nshared);
>>
>>          if (!QLIST_EMPTY(&bs->parents)) {
>>              if (perm & BLK_PERM_WRITE) {
>>                  *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
>>              }
>>              *nshared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
>>          }
> 
> So I wasn't sure how to adapt the patch:
> 
> - If setting BLK_PERM_WRITE unconditionally, it seems to break usual
> drive-backup (with no fleecing set up):
> 
>> permissions 'write' are both required by node '#block691' (uses node '#block151' as 'file' child) and unshared by block device 'drive-scsi0' (uses node '#block151' as 'root' child).
> 
> - If I only do it within the if block, it doesn't work when I try to set
> up fleecing, because bs->parents is empty for me, i.e. when passing the
> snapshot-access node to backup_job_create() while the usual cbw for
> backup is appended. I should note I'm doing it manually in a custom QMP
> command, not in a transaction (which requires the not-yet-merged
> blockdev-replace AFAIU).
> 
> Not sure if I'm doing something wrong, but maybe what you wrote in the
> commit message is necessary after all?
> 
>> Alternative is to pass
>> an option to bdrv_cbw_append(), add some internal open-option for
>> copy-before-write filter to require WRITE permission only for backup
>> with discard-source=true. But I'm not sure it worth the complexity.
> 
> Best Regards,
> Fiona
>