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>
---
block/backup.c | 5 +++--
block/block-copy.c | 12 ++++++++++--
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, 23 insertions(+), 10 deletions(-)
diff --git a/block/backup.c b/block/backup.c
index ec29d6b810..0c86b5be55 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -356,7 +356,7 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
BlockDriverState *target, int64_t speed,
MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
BitmapSyncMode bitmap_mode,
- bool compress,
+ bool compress, bool discard_source,
const char *filter_node_name,
BackupPerf *perf,
BlockdevOnError on_source_error,
@@ -491,7 +491,8 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
job->len = len;
job->perf = *perf;
- block_copy_set_copy_opts(bcs, perf->use_copy_range, compress);
+ block_copy_set_copy_opts(bcs, perf->use_copy_range, compress,
+ discard_source);
block_copy_set_progress_meter(bcs, &job->common.job.progress);
block_copy_set_speed(bcs, speed);
diff --git a/block/block-copy.c b/block/block-copy.c
index 8fca2c3698..eebf643e86 100644
--- a/block/block-copy.c
+++ b/block/block-copy.c
@@ -137,6 +137,7 @@ typedef struct BlockCopyState {
CoMutex lock;
int64_t in_flight_bytes;
BlockCopyMethod method;
+ bool discard_source;
BlockReqList reqs;
QLIST_HEAD(, BlockCopyCallState) calls;
/*
@@ -282,11 +283,12 @@ static uint32_t block_copy_max_transfer(BdrvChild *source, BdrvChild *target)
}
void block_copy_set_copy_opts(BlockCopyState *s, bool use_copy_range,
- bool compress)
+ bool compress, bool discard_source)
{
/* Keep BDRV_REQ_SERIALISING set (or not set) in block_copy_state_new() */
s->write_flags = (s->write_flags & BDRV_REQ_SERIALISING) |
(compress ? BDRV_REQ_WRITE_COMPRESSED : 0);
+ s->discard_source = discard_source;
if (s->max_transfer < s->cluster_size) {
/*
@@ -418,7 +420,7 @@ BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
cluster_size),
};
- block_copy_set_copy_opts(s, false, false);
+ block_copy_set_copy_opts(s, false, false, false);
ratelimit_init(&s->rate_limit);
qemu_co_mutex_init(&s->lock);
@@ -589,6 +591,12 @@ 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) {
+ int64_t nbytes =
+ MIN(t->req.offset + t->req.bytes, s->len) - t->req.offset;
+ bdrv_co_pdiscard(s->source, t->req.offset, nbytes);
+ }
+
return ret;
}
diff --git a/block/copy-before-write.c b/block/copy-before-write.c
index 4cd9b7d91e..b208a80ade 100644
--- a/block/copy-before-write.c
+++ b/block/copy-before-write.c
@@ -370,7 +370,7 @@ cbw_child_perm(BlockDriverState *bs, BdrvChild *c, BdrvChildRole role,
* works even if source node does not have any parents before backup
* start
*/
- *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
+ *nperm = *nperm | BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE;
*nshared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
}
}
diff --git a/block/replication.c b/block/replication.c
index ca6bd0a720..0415a5e8b7 100644
--- a/block/replication.c
+++ b/block/replication.c
@@ -582,8 +582,8 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
s->backup_job = backup_job_create(
NULL, s->secondary_disk->bs, s->hidden_disk->bs,
- 0, MIRROR_SYNC_MODE_NONE, NULL, 0, false, NULL,
- &perf,
+ 0, MIRROR_SYNC_MODE_NONE, NULL, 0, false, false,
+ NULL, &perf,
BLOCKDEV_ON_ERROR_REPORT,
BLOCKDEV_ON_ERROR_REPORT, JOB_INTERNAL,
backup_job_completed, bs, NULL, &local_err);
diff --git a/blockdev.c b/blockdev.c
index 3a5e7222ec..aa64382f82 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -2713,7 +2713,7 @@ static BlockJob *do_backup_common(BackupCommon *backup,
job = backup_job_create(backup->job_id, bs, target_bs, backup->speed,
backup->sync, bmap, backup->bitmap_mode,
- backup->compress,
+ backup->compress, backup->discard_source,
backup->filter_node_name,
&perf,
backup->on_source_error,
diff --git a/include/block/block-copy.h b/include/block/block-copy.h
index 8b41643bfa..9555de2562 100644
--- a/include/block/block-copy.h
+++ b/include/block/block-copy.h
@@ -31,7 +31,7 @@ BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
/* Function should be called prior any actual copy request */
void block_copy_set_copy_opts(BlockCopyState *s, bool use_copy_range,
- bool compress);
+ bool compress, bool discard_source);
void block_copy_set_progress_meter(BlockCopyState *s, ProgressMeter *pm);
void block_copy_state_free(BlockCopyState *s);
diff --git a/include/block/block_int-global-state.h b/include/block/block_int-global-state.h
index ef31c58bb3..563aabf012 100644
--- a/include/block/block_int-global-state.h
+++ b/include/block/block_int-global-state.h
@@ -187,7 +187,7 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
MirrorSyncMode sync_mode,
BdrvDirtyBitmap *sync_bitmap,
BitmapSyncMode bitmap_mode,
- bool compress,
+ bool compress, bool discard_source,
const char *filter_node_name,
BackupPerf *perf,
BlockdevOnError on_source_error,
diff --git a/qapi/block-core.json b/qapi/block-core.json
index ca390c5700..1735769f9f 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1599,6 +1599,9 @@
# node specified by @drive. If this option is not given, a node
# name is autogenerated. (Since: 4.2)
#
+# @discard-source: Discard blocks on source which are already copied
+# to the target. (Since 9.0)
+#
# @x-perf: Performance options. (Since 6.0)
#
# Features:
@@ -1620,6 +1623,7 @@
'*on-target-error': 'BlockdevOnError',
'*auto-finalize': 'bool', '*auto-dismiss': 'bool',
'*filter-node-name': 'str',
+ '*discard-source': 'bool',
'*x-perf': { 'type': 'BackupPerf',
'features': [ 'unstable' ] } } }
--
2.34.1Am 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>
Ran into another issue when the cluster_size of the fleecing image is
larger than for the backup target, e.g.
> #!/bin/bash
> rm /tmp/fleecing.qcow2
> ./qemu-img create /tmp/disk.qcow2 -f qcow2 1G
> ./qemu-img create /tmp/fleecing.qcow2 -o cluster_size=2M -f qcow2 1G
> ./qemu-img create /tmp/backup.qcow2 -f qcow2 1G
> ./qemu-system-x86_64 --qmp stdio \
> --blockdev qcow2,node-name=node0,file.driver=file,file.filename=/tmp/disk.qcow2 \
> --blockdev qcow2,node-name=node1,file.driver=file,file.filename=/tmp/fleecing.qcow2,discard=unmap \
> --blockdev qcow2,node-name=node2,file.driver=file,file.filename=/tmp/backup.qcow2 \
> <<EOF
> {"execute": "qmp_capabilities"}
> {"execute": "blockdev-add", "arguments": { "driver": "copy-before-write", "file": "node0", "target": "node1", "node-name": "node3" } }
> {"execute": "blockdev-add", "arguments": { "driver": "snapshot-access", "file": "node3", "discard": "unmap", "node-name": "snap0" } }
> {"execute": "blockdev-backup", "arguments": { "device": "snap0", "target": "node2", "sync": "full", "job-id": "backup0", "discard-source": true } }
> EOF
will fail with
> qemu-system-x86_64: ../util/hbitmap.c:570: hbitmap_reset: Assertion `QEMU_IS_ALIGNED(count, gran) || (start + count == hb->orig_size)' failed.
Backtrace shows the assert happens while discarding, when resetting the
BDRVCopyBeforeWriteState access_bitmap
> #6 0x0000555556142a2a in hbitmap_reset (hb=0x555557e01b80, start=0,
count=1048576) at ../util/hbitmap.c:570
> #7 0x0000555555f80764 in bdrv_reset_dirty_bitmap_locked (bitmap=0x55555850a660, offset=0, bytes=1048576) at ../block/dirty-bitmap.c:563
> #8 0x0000555555f807ab in bdrv_reset_dirty_bitmap (bitmap=0x55555850a660, offset=0, bytes=1048576) at ../block/dirty-bitmap.c:570
> #9 0x0000555555f7bb16 in cbw_co_pdiscard_snapshot (bs=0x5555581a7f60, offset=0, bytes=1048576) at ../block/copy-before-write.c:330
> #10 0x0000555555f8d00a in bdrv_co_pdiscard_snapshot (bs=0x5555581a7f60, offset=0, bytes=1048576) at ../block/io.c:3734
> #11 0x0000555555fd2380 in snapshot_access_co_pdiscard (bs=0x5555582b4f60, offset=0, bytes=1048576) at ../block/snapshot-access.c:55
> #12 0x0000555555f8b65d in bdrv_co_pdiscard (child=0x5555584fe790, offset=0, bytes=1048576) at ../block/io.c:3144
> #13 0x0000555555f78650 in block_copy_task_entry (task=0x555557f588f0) at ../block/block-copy.c:597
My guess for the cause is that in block_copy_calculate_cluster_size() we
only look at the target. But now that we need to discard the source,
we'll also need to consider that for the calculation?
Best Regards,
Fiona
Am 24.01.24 um 16:03 schrieb Fiona Ebner:
> 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>
>
> Ran into another issue when the cluster_size of the fleecing image is
> larger than for the backup target, e.g.
>
>> #!/bin/bash
>> rm /tmp/fleecing.qcow2
>> ./qemu-img create /tmp/disk.qcow2 -f qcow2 1G
>> ./qemu-img create /tmp/fleecing.qcow2 -o cluster_size=2M -f qcow2 1G
>> ./qemu-img create /tmp/backup.qcow2 -f qcow2 1G
>> ./qemu-system-x86_64 --qmp stdio \
>> --blockdev qcow2,node-name=node0,file.driver=file,file.filename=/tmp/disk.qcow2 \
>> --blockdev qcow2,node-name=node1,file.driver=file,file.filename=/tmp/fleecing.qcow2,discard=unmap \
>> --blockdev qcow2,node-name=node2,file.driver=file,file.filename=/tmp/backup.qcow2 \
>> <<EOF
>> {"execute": "qmp_capabilities"}
>> {"execute": "blockdev-add", "arguments": { "driver": "copy-before-write", "file": "node0", "target": "node1", "node-name": "node3" } }
>> {"execute": "blockdev-add", "arguments": { "driver": "snapshot-access", "file": "node3", "discard": "unmap", "node-name": "snap0" } }
>> {"execute": "blockdev-backup", "arguments": { "device": "snap0", "target": "node2", "sync": "full", "job-id": "backup0", "discard-source": true } }
>> EOF
>
> will fail with
>
>> qemu-system-x86_64: ../util/hbitmap.c:570: hbitmap_reset: Assertion `QEMU_IS_ALIGNED(count, gran) || (start + count == hb->orig_size)' failed.
>
> Backtrace shows the assert happens while discarding, when resetting the
> BDRVCopyBeforeWriteState access_bitmap
> > #6 0x0000555556142a2a in hbitmap_reset (hb=0x555557e01b80, start=0,
> count=1048576) at ../util/hbitmap.c:570
>> #7 0x0000555555f80764 in bdrv_reset_dirty_bitmap_locked (bitmap=0x55555850a660, offset=0, bytes=1048576) at ../block/dirty-bitmap.c:563
>> #8 0x0000555555f807ab in bdrv_reset_dirty_bitmap (bitmap=0x55555850a660, offset=0, bytes=1048576) at ../block/dirty-bitmap.c:570
>> #9 0x0000555555f7bb16 in cbw_co_pdiscard_snapshot (bs=0x5555581a7f60, offset=0, bytes=1048576) at ../block/copy-before-write.c:330
>> #10 0x0000555555f8d00a in bdrv_co_pdiscard_snapshot (bs=0x5555581a7f60, offset=0, bytes=1048576) at ../block/io.c:3734
>> #11 0x0000555555fd2380 in snapshot_access_co_pdiscard (bs=0x5555582b4f60, offset=0, bytes=1048576) at ../block/snapshot-access.c:55
>> #12 0x0000555555f8b65d in bdrv_co_pdiscard (child=0x5555584fe790, offset=0, bytes=1048576) at ../block/io.c:3144
>> #13 0x0000555555f78650 in block_copy_task_entry (task=0x555557f588f0) at ../block/block-copy.c:597
>
> My guess for the cause is that in block_copy_calculate_cluster_size() we
> only look at the target. But now that we need to discard the source,
> we'll also need to consider that for the calculation?
>
Just querying the source and picking the maximum won't work either,
because snapshot-access does not currently implement .bdrv_co_get_info
and because copy-before-write (doesn't implement .bdrv_co_get_info and
is a filter) will just return the info of its file child. But the
discard will go to the target child.
If I do
1. .bdrv_co_get_info in snapshot-access: return info from file child
2. .bdrv_co_get_info in copy-before-write: return maximum cluster_size
from file child and target child
3. block_copy_calculate_cluster_size: return maximum from source and target
then the issue does go away, but I don't know if that's not violating
any assumptions and probably there's a better way to avoid the issue?
Best Regards,
Fiona
On 25.01.24 15:47, Fiona Ebner wrote:
> Am 24.01.24 um 16:03 schrieb Fiona Ebner:
>> 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>
>>
>> Ran into another issue when the cluster_size of the fleecing image is
>> larger than for the backup target, e.g.
>>
>>> #!/bin/bash
>>> rm /tmp/fleecing.qcow2
>>> ./qemu-img create /tmp/disk.qcow2 -f qcow2 1G
>>> ./qemu-img create /tmp/fleecing.qcow2 -o cluster_size=2M -f qcow2 1G
>>> ./qemu-img create /tmp/backup.qcow2 -f qcow2 1G
>>> ./qemu-system-x86_64 --qmp stdio \
>>> --blockdev qcow2,node-name=node0,file.driver=file,file.filename=/tmp/disk.qcow2 \
>>> --blockdev qcow2,node-name=node1,file.driver=file,file.filename=/tmp/fleecing.qcow2,discard=unmap \
>>> --blockdev qcow2,node-name=node2,file.driver=file,file.filename=/tmp/backup.qcow2 \
>>> <<EOF
>>> {"execute": "qmp_capabilities"}
>>> {"execute": "blockdev-add", "arguments": { "driver": "copy-before-write", "file": "node0", "target": "node1", "node-name": "node3" } }
>>> {"execute": "blockdev-add", "arguments": { "driver": "snapshot-access", "file": "node3", "discard": "unmap", "node-name": "snap0" } }
>>> {"execute": "blockdev-backup", "arguments": { "device": "snap0", "target": "node2", "sync": "full", "job-id": "backup0", "discard-source": true } }
>>> EOF
>>
>> will fail with
>>
>>> qemu-system-x86_64: ../util/hbitmap.c:570: hbitmap_reset: Assertion `QEMU_IS_ALIGNED(count, gran) || (start + count == hb->orig_size)' failed.
>>
>> Backtrace shows the assert happens while discarding, when resetting the
>> BDRVCopyBeforeWriteState access_bitmap
>> > #6 0x0000555556142a2a in hbitmap_reset (hb=0x555557e01b80, start=0,
>> count=1048576) at ../util/hbitmap.c:570
>>> #7 0x0000555555f80764 in bdrv_reset_dirty_bitmap_locked (bitmap=0x55555850a660, offset=0, bytes=1048576) at ../block/dirty-bitmap.c:563
>>> #8 0x0000555555f807ab in bdrv_reset_dirty_bitmap (bitmap=0x55555850a660, offset=0, bytes=1048576) at ../block/dirty-bitmap.c:570
>>> #9 0x0000555555f7bb16 in cbw_co_pdiscard_snapshot (bs=0x5555581a7f60, offset=0, bytes=1048576) at ../block/copy-before-write.c:330
>>> #10 0x0000555555f8d00a in bdrv_co_pdiscard_snapshot (bs=0x5555581a7f60, offset=0, bytes=1048576) at ../block/io.c:3734
>>> #11 0x0000555555fd2380 in snapshot_access_co_pdiscard (bs=0x5555582b4f60, offset=0, bytes=1048576) at ../block/snapshot-access.c:55
>>> #12 0x0000555555f8b65d in bdrv_co_pdiscard (child=0x5555584fe790, offset=0, bytes=1048576) at ../block/io.c:3144
>>> #13 0x0000555555f78650 in block_copy_task_entry (task=0x555557f588f0) at ../block/block-copy.c:597
>>
>> My guess for the cause is that in block_copy_calculate_cluster_size() we
>> only look at the target. But now that we need to discard the source,
>> we'll also need to consider that for the calculation?
>>
>
> Just querying the source and picking the maximum won't work either,
> because snapshot-access does not currently implement .bdrv_co_get_info
> and because copy-before-write (doesn't implement .bdrv_co_get_info and
> is a filter) will just return the info of its file child. But the
> discard will go to the target child.
>
> If I do
>
> 1. .bdrv_co_get_info in snapshot-access: return info from file child
> 2. .bdrv_co_get_info in copy-before-write: return maximum cluster_size
> from file child and target child
> 3. block_copy_calculate_cluster_size: return maximum from source and target
>
> then the issue does go away, but I don't know if that's not violating
> any assumptions and probably there's a better way to avoid the issue?
>
Hmm. Taking maximum is not optimal for usual case without discard-source: user may want to work in smaller granularity than source, to save disk space.
In case with discarding we have two possibilities:
- either take larger granularity for the whole process like you propose (but this will need and option for CBW?)
- or, fix discarding bitmap in CBW to work like normal discard: it should be aligned down. This will lead actually to discard-source option doing nothing..
==
But why do you want fleecing image with larger granularity? Is that a real case or just experimenting? Still we should fix assertion anyway.
I think:
1. fix discarding bitmap to make aligning-down (will do that for v3)
2. if we need another logic for block_copy_calculate_cluster_size() it should be an option. May be explicit "copy-cluster-size" or "granularity" option for CBW driver and for backup job. And we'll just check that given cluster-size is power of two >= target_size.
--
Best regards,
Vladimir
Am 25.01.24 um 18:22 schrieb Vladimir Sementsov-Ogievskiy: > > Hmm. Taking maximum is not optimal for usual case without > discard-source: user may want to work in smaller granularity than > source, to save disk space. > > In case with discarding we have two possibilities: > > - either take larger granularity for the whole process like you propose > (but this will need and option for CBW?) > - or, fix discarding bitmap in CBW to work like normal discard: it > should be aligned down. This will lead actually to discard-source option > doing nothing.. > > == > But why do you want fleecing image with larger granularity? Is that a > real case or just experimenting? Still we should fix assertion anyway. > Yes, it's a real use case. We do support different storage types and want to allow users to place the fleecing image on a different storage than the original image for flexibility. I ran into the issue when backing up to a target with 1 MiB cluster_size while using a fleecing image on RBD (which has 4 MiB cluster_size by default). In theory, I guess I could look into querying the cluster_size of the backup target and trying to allocate the fleecing image with a small enough cluster_size. But not sure if that would work on all storage combinations, and would require teaching our storage plugin API (which also supports third-party plugins) to perform allocation with a specific cluster size. So not an ideal solution for us. > I think: > > 1. fix discarding bitmap to make aligning-down (will do that for v3) > Thanks! > 2. if we need another logic for block_copy_calculate_cluster_size() it > should be an option. May be explicit "copy-cluster-size" or > "granularity" option for CBW driver and for backup job. And we'll just > check that given cluster-size is power of two >= target_size. > I'll try to implement point 2. That should resolve the issue for our use case. Best Regards, Fiona
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):
> #!/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
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
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