qemu-io: add drain/undrain cmd

Hi Block developers,

I would like to add a feature to Qemu to drain all traffic from a block so that I can take external snaphosts without the risk to that in the middle of a write operation. Its meant for cases where where QGA freeze/thaw is not available.

For me its enough to have this through qemu-io, but Kevin asked me to check if its not worth to have a stable API for it and present it via QMP/HMP.

What are your thoughts?

Thanks,
Peter

---
 qemu-io-cmds.c | 78 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 78 insertions(+)

qemu-io-cmds.c +78 -0

index 312fc6d..49d82fe 100644
@@ -1565,6 +1565,82 @@ static const cmdinfo_t flush_cmd = {
     .oneline    = "flush all in-core file state to disk",
 };
 
+static const cmdinfo_t drain_cmd;
+
+static int drain_f(BlockBackend *blk, int argc, char **argv)
+{
+    BlockDriverState *bs = blk_bs(blk);
+    bool flush = false;
+    int c;
+
+    while ((c = getopt(argc, argv, "f")) != -1) {
+        switch (c) {
+        case 'f':
+            flush = true;
+            break;
+        default:
+            return qemuio_command_usage(&drain_cmd);
+        }
+    }
+
+    if (optind != argc) {
+        return qemuio_command_usage(&drain_cmd);
+    }
+
+
+    if (bs->quiesce_counter) {
+        printf("drain failed: device is already drained!\n");
+        return 1;
+    }
+
+    bdrv_drained_begin(bs); /* complete I/O */
+    if (flush) {
+        bdrv_flush(bs);
+        bdrv_drain(bs); /* in case flush left pending I/O */
+        printf("flushed all pending I/O\n");
+    }
+    printf("drain successful\n");
+    return 0;
+}
+
+static void drain_help(void)
+{
+    printf(
+"\n"
+" Drains all external I/O from the device\n"
+"\n"
+" -f, -- flush all in-core file state to disk\n"
+"\n");
+}
+
+static const cmdinfo_t drain_cmd = {
+    .name       = "drain",
+    .cfunc      = drain_f,
+    .args       = "[-f]",
+    .argmin     = 0,
+    .argmax     = -1,
+    .oneline    = "cease to send I/O to the device",
+    .help       = drain_help
+};
+
+static int undrain_f(BlockBackend *blk, int argc, char **argv)
+{
+    BlockDriverState *bs = blk_bs(blk);
+    if (!bs->quiesce_counter) {
+        printf("undrain failed: device is not drained!\n");
+        return 1;
+    }
+    bdrv_drained_end(bs);
+    printf("undrain successful\n");
+    return 0;
+}
+
+static const cmdinfo_t undrain_cmd = {
+    .name       = "undrain",
+    .cfunc      = undrain_f,
+    .oneline    = "continue I/O to a drained device",
+};
+
 static int truncate_f(BlockBackend *blk, int argc, char **argv)
 {
     int64_t offset;

@@ -2296,6 +2372,8 @@ static void __attribute((constructor)) init_qemuio_commands(void)
     qemuio_add_command(&aio_write_cmd);
     qemuio_add_command(&aio_flush_cmd);
     qemuio_add_command(&flush_cmd);
+    qemuio_add_command(&drain_cmd);
+    qemuio_add_command(&undrain_cmd);
     qemuio_add_command(&truncate_cmd);
     qemuio_add_command(&length_cmd);
     qemuio_add_command(&info_cmd);

Signature · 1 line

1.9.1

Quoted 6 lines

Hi Block developers,

I would like to add a feature to Qemu to drain all traffic from a block so that I can take external snaphosts without the risk to that in the middle of a write operation. Its meant for cases where where QGA freeze/thaw is not available.

Quoted 4 lines

For me its enough to have this through qemu-io, but Kevin asked me to check if its not worth to have a stable API for it and present it via QMP/HMP.

What are your thoughts?

For debugging purpose or a "hacky" usage where you know what you are doing, it may be fine to have this. The only issue is it should be a separate flag, like BlockJob.user_paused.

What happens from guest perspective? In the case of virtio, the request queue is not handled and -ETIMEDOUT may happen. With IDE, I/O commands are still handled, the command is not effective (or rather the implementation is not complete).

Am 15.05.2017 um 12:50 schrieb Fam Zheng:

Quoted 10 lines · Peter Lieven

Quoted 9 lines

Hi Block developers,

I would like to add a feature to Qemu to drain all traffic from a block so that I can take external snaphosts without the risk to that in the middle of a write operation. Its meant for cases where where QGA freeze/thaw is not available.

For me its enough to have this through qemu-io, but Kevin asked me to check if its not worth to have a stable API for it and present it via QMP/HMP.

Quoted 4 lines

Quoted 1 line

What are your thoughts?

For debugging purpose or a "hacky" usage where you know what you are doing, it may be fine to have this. The only issue is it should be a separate flag, like BlockJob.user_paused.

How can I add, remove such a flag?

Quoted 5 lines

What happens from guest perspective? In the case of virtio, the request queue is not handled and -ETIMEDOUT may happen. With IDE, I/O commands are still handled, the command is not effective (or rather the implementation is not complete).

That it only works with virtio is fine. However, the thing it does not work correctly apply then also to all other users of the drained_begin/end functions, right? As for the timeout I only plan to drain the device for about 1 second.

Quoted 11 lines

Am 15.05.2017 um 12:50 schrieb Fam Zheng:

Quoted 10 lines · Peter Lieven

Quoted 9 lines

Hi Block developers,

I would like to add a feature to Qemu to drain all traffic from a block so that I can take external snaphosts without the risk to that in the middle of a write operation. Its meant for cases where where QGA freeze/thaw is not available.

For me its enough to have this through qemu-io, but Kevin asked me to check if its not worth to have a stable API for it and present it via QMP/HMP.

Quoted 5 lines

Quoted 4 lines

Quoted 1 line

What are your thoughts?

For debugging purpose or a "hacky" usage where you know what you are doing, it may be fine to have this. The only issue is it should be a separate flag, like BlockJob.user_paused.

How can I add, remove such a flag?

Like bs->user_drained. Set it in "drain" command, then increment bs->quiesce_counter if toggled; vise versa.

Quoted 7 lines

Quoted 4 lines

What happens from guest perspective? In the case of virtio, the request queue is not handled and -ETIMEDOUT may happen. With IDE, I/O commands are still handled, the command is not effective (or rather the implementation is not complete).

That it only works with virtio is fine. However, the thing it does not work correctly apply then also to all other users of the drained_begin/end functions, right? As for the timeout I only plan to drain the device for about 1 second.

It didn't matter because for IDE, the invariant (staying quiesced as long as necessary) is already ensured by BQL. Virtio is different because it supports ioeventfd and data plane.

Am 15.05.2017 um 13:53 schrieb Fam Zheng:

Quoted 16 lines · Peter Lieven

Quoted 15 lines

Am 15.05.2017 um 12:50 schrieb Fam Zheng:

Quoted 14 lines · Peter Lieven

Quoted 10 lines

Hi Block developers,

I would like to add a feature to Qemu to drain all traffic from a block so that I can take external snaphosts without the risk to that in the middle of a write operation. Its meant for cases where where QGA freeze/thaw is not available.

For me its enough to have this through qemu-io, but Kevin asked me to check if its not worth to have a stable API for it and present it via QMP/HMP.

What are your thoughts?

For debugging purpose or a "hacky" usage where you know what you are doing, it may be fine to have this. The only issue is it should be a separate flag, like BlockJob.user_paused.

Quoted 3 lines

Quoted 1 line

How can I add, remove such a flag?

Like bs->user_drained. Set it in "drain" command, then increment bs->quiesce_counter if toggled; vise versa.

Ah okay. You wouldn't use bdrv_drained_begin/end? Because in these functions the counter is incremented already.

Quoted 10 lines

Quoted 10 lines

Quoted 7 lines

What happens from guest perspective? In the case of virtio, the request queue is not handled and -ETIMEDOUT may happen. With IDE, I/O commands are still handled, the command is not effective (or rather the implementation is not complete).

That it only works with virtio is fine. However, the thing it does not work correctly apply then also to all other users of the drained_begin/end functions, right? As for the timeout I only plan to drain the device for about 1 second.

Quoted 3 lines

It didn't matter because for IDE, the invariant (staying quiesced as long as necessary) is already ensured by BQL. Virtio is different because it supports ioeventfd and data plane.

Okay understood. So my use of bdrv_drained_begin/end is more an abuse of these functions?

Do you have another idea how to achieve what I want? I was thinking of throttle the I/O to zero. It would be enough to do this for writes, reading doesn't hurt in my case.

Quoted 14 lines

Am 15.05.2017 um 13:53 schrieb Fam Zheng:

Quoted 13 lines · Peter Lieven

Quoted 12 lines

Am 15.05.2017 um 12:50 schrieb Fam Zheng:

Quoted 11 lines · Peter Lieven

Quoted 10 lines

Hi Block developers,

I would like to add a feature to Qemu to drain all traffic from a block so that I can take external snaphosts without the risk to that in the middle of a write operation. Its meant for cases where where QGA freeze/thaw is not available.

For me its enough to have this through qemu-io, but Kevin asked me to check if its not worth to have a stable API for it and present it via QMP/HMP.

What are your thoughts?

Quoted 8 lines

Quoted 6 lines

Quoted 4 lines

Quoted 3 lines

For debugging purpose or a "hacky" usage where you know what you are doing, it may be fine to have this. The only issue is it should be a separate flag, like BlockJob.user_paused.

How can I add, remove such a flag?

Like bs->user_drained. Set it in "drain" command, then increment bs->quiesce_counter if toggled; vise versa.

Ah okay. You wouldn't use bdrv_drained_begin/end? Because in these functions the counter is incremented already.

You're right, calling bdrv_drained_begin() is better.

Quoted 5 lines

Quoted 5 lines

Quoted 5 lines

Quoted 5 lines

What happens from guest perspective? In the case of virtio, the request queue is not handled and -ETIMEDOUT may happen. With IDE, I/O commands are still handled, the command is not effective (or rather the implementation is not complete).

Quoted 8 lines

Quoted 6 lines

Quoted 3 lines

That it only works with virtio is fine. However, the thing it does not work correctly apply then also to all other users of the drained_begin/end functions, right? As for the timeout I only plan to drain the device for about 1 second.

It didn't matter because for IDE, the invariant (staying quiesced as long as necessary) is already ensured by BQL. Virtio is different because it supports ioeventfd and data plane.

Okay understood. So my use of bdrv_drained_begin/end is more an abuse of these functions?

Sort of. But it's not unreasonable to "extend" bdrv_drained_begin/end to cover IDE, I just haven't thought about "how".

Quoted 3 lines

Do you have another idea how to achieve what I want? I was thinking of throttle the I/O to zero. It would be enough to do this for writes, reading doesn't hurt in my case.

Maybe add a block filter on top of the drained node, drain it when doing so, then queue all further requests with a CoQueue until "undrain". (It is then not quite to "drain" but to "halt" or "pause", though.)

Am 15.05.2017 um 14:28 schrieb Fam Zheng:

Quoted 44 lines · Peter Lieven

Quoted 14 lines

Am 15.05.2017 um 13:53 schrieb Fam Zheng:

Quoted 13 lines · Peter Lieven

Quoted 12 lines

Am 15.05.2017 um 12:50 schrieb Fam Zheng:

Quoted 11 lines · Peter Lieven

Quoted 10 lines

Hi Block developers,

I would like to add a feature to Qemu to drain all traffic from a block so that I can take external snaphosts without the risk to that in the middle of a write operation. Its meant for cases where where QGA freeze/thaw is not available.

For me its enough to have this through qemu-io, but Kevin asked me to check if its not worth to have a stable API for it and present it via QMP/HMP.

What are your thoughts?

Quoted 8 lines

Quoted 6 lines

Quoted 4 lines

Quoted 3 lines

For debugging purpose or a "hacky" usage where you know what you are doing, it may be fine to have this. The only issue is it should be a separate flag, like BlockJob.user_paused.

How can I add, remove such a flag?

Like bs->user_drained. Set it in "drain" command, then increment bs->quiesce_counter if toggled; vise versa.

Ah okay. You wouldn't use bdrv_drained_begin/end? Because in these functions the counter is incremented already.

You're right, calling bdrv_drained_begin() is better.

Quoted 5 lines

Quoted 5 lines

Quoted 5 lines

Quoted 5 lines

What happens from guest perspective? In the case of virtio, the request queue is not handled and -ETIMEDOUT may happen. With IDE, I/O commands are still handled, the command is not effective (or rather the implementation is not complete).

Quoted 8 lines

Quoted 6 lines

Quoted 3 lines

That it only works with virtio is fine. However, the thing it does not work correctly apply then also to all other users of the drained_begin/end functions, right? As for the timeout I only plan to drain the device for about 1 second.

It didn't matter because for IDE, the invariant (staying quiesced as long as necessary) is already ensured by BQL. Virtio is different because it supports ioeventfd and data plane.

Okay understood. So my use of bdrv_drained_begin/end is more an abuse of these functions?

Sort of. But it's not unreasonable to "extend" bdrv_drained_begin/end to cover IDE, I just haven't thought about "how".

Quoted 3 lines

Do you have another idea how to achieve what I want? I was thinking of throttle the I/O to zero. It would be enough to do this for writes, reading doesn't hurt in my case.

Quoted 3 lines

Maybe add a block filter on top of the drained node, drain it when doing so, then queue all further requests with a CoQueue until "undrain". (It is then not quite to "drain" but to "halt" or "pause", though.)

To get the drain for free was why I was looking at this approach. If I read correctly if I keep using bdrv_drained_begin/end its too hacky to implement it in QMP?

If yes, would support adding it to qemu-io?

Quoted 41 lines

Am 15.05.2017 um 14:28 schrieb Fam Zheng:

Quoted 40 lines · Peter Lieven

Quoted 14 lines

Am 15.05.2017 um 13:53 schrieb Fam Zheng:

Quoted 13 lines · Peter Lieven

Quoted 12 lines

Am 15.05.2017 um 12:50 schrieb Fam Zheng:

Quoted 11 lines · Peter Lieven

Quoted 10 lines

Hi Block developers,

I would like to add a feature to Qemu to drain all traffic from a block so that I can take external snaphosts without the risk to that in the middle of a write operation. Its meant for cases where where QGA freeze/thaw is not available.

For me its enough to have this through qemu-io, but Kevin asked me to check if its not worth to have a stable API for it and present it via QMP/HMP.

What are your thoughts?

Quoted 8 lines

Quoted 6 lines

Quoted 4 lines

Quoted 3 lines

For debugging purpose or a "hacky" usage where you know what you are doing, it may be fine to have this. The only issue is it should be a separate flag, like BlockJob.user_paused.

How can I add, remove such a flag?

Like bs->user_drained. Set it in "drain" command, then increment bs->quiesce_counter if toggled; vise versa.

Ah okay. You wouldn't use bdrv_drained_begin/end? Because in these functions the counter is incremented already.

You're right, calling bdrv_drained_begin() is better.

Quoted 4 lines

Quoted 4 lines

Quoted 4 lines

Quoted 4 lines

What happens from guest perspective? In the case of virtio, the request queue is not handled and -ETIMEDOUT may happen. With IDE, I/O commands are still handled, the command is not effective (or rather the implementation is not complete).

Quoted 8 lines

Quoted 6 lines

Quoted 3 lines

That it only works with virtio is fine. However, the thing it does not work correctly apply then also to all other users of the drained_begin/end functions, right? As for the timeout I only plan to drain the device for about 1 second.

It didn't matter because for IDE, the invariant (staying quiesced as long as necessary) is already ensured by BQL. Virtio is different because it supports ioeventfd and data plane.

Okay understood. So my use of bdrv_drained_begin/end is more an abuse of these functions?

Sort of. But it's not unreasonable to "extend" bdrv_drained_begin/end to cover IDE, I just haven't thought about "how".

Quoted 8 lines

Quoted 6 lines

Quoted 3 lines

Do you have another idea how to achieve what I want? I was thinking of throttle the I/O to zero. It would be enough to do this for writes, reading doesn't hurt in my case.

Maybe add a block filter on top of the drained node, drain it when doing so, then queue all further requests with a CoQueue until "undrain". (It is then not quite to "drain" but to "halt" or "pause", though.)

To get the drain for free was why I was looking at this approach. If I read correctly if I keep using bdrv_drained_begin/end its too hacky to implement it in QMP?

I think so.

Quoted 1 line

If yes, would support adding it to qemu-io?

I'm under the impression that you are looking to a real use case, I don't think I like the idea. Also, accessing the image from other processes while QEMU is using it is strongly discouraged, and there is the coming image locking mechanism to prevent this from happening. Why is the blockdev-snapshot command not enough?

Am 15.05.2017 um 14:52 schrieb Fam Zheng:

Quoted 50 lines · Peter Lieven

Quoted 41 lines

Am 15.05.2017 um 14:28 schrieb Fam Zheng:

Quoted 40 lines · Peter Lieven

Quoted 14 lines

Am 15.05.2017 um 13:53 schrieb Fam Zheng:

Quoted 13 lines · Peter Lieven

Quoted 12 lines

Am 15.05.2017 um 12:50 schrieb Fam Zheng:

Quoted 11 lines · Peter Lieven

Quoted 10 lines

Hi Block developers,

I would like to add a feature to Qemu to drain all traffic from a block so that I can take external snaphosts without the risk to that in the middle of a write operation. Its meant for cases where where QGA freeze/thaw is not available.

For me its enough to have this through qemu-io, but Kevin asked me to check if its not worth to have a stable API for it and present it via QMP/HMP.

What are your thoughts?

Quoted 8 lines

Quoted 6 lines

Quoted 4 lines

Quoted 3 lines

For debugging purpose or a "hacky" usage where you know what you are doing, it may be fine to have this. The only issue is it should be a separate flag, like BlockJob.user_paused.

How can I add, remove such a flag?

Like bs->user_drained. Set it in "drain" command, then increment bs->quiesce_counter if toggled; vise versa.

Ah okay. You wouldn't use bdrv_drained_begin/end? Because in these functions the counter is incremented already.

You're right, calling bdrv_drained_begin() is better.

Quoted 4 lines

Quoted 4 lines

Quoted 4 lines

Quoted 4 lines

What happens from guest perspective? In the case of virtio, the request queue is not handled and -ETIMEDOUT may happen. With IDE, I/O commands are still handled, the command is not effective (or rather the implementation is not complete).

Quoted 8 lines

Quoted 6 lines

Quoted 3 lines

That it only works with virtio is fine. However, the thing it does not work correctly apply then also to all other users of the drained_begin/end functions, right? As for the timeout I only plan to drain the device for about 1 second.

It didn't matter because for IDE, the invariant (staying quiesced as long as necessary) is already ensured by BQL. Virtio is different because it supports ioeventfd and data plane.

Okay understood. So my use of bdrv_drained_begin/end is more an abuse of these functions?

Sort of. But it's not unreasonable to "extend" bdrv_drained_begin/end to cover IDE, I just haven't thought about "how".

Quoted 8 lines

Quoted 6 lines

Quoted 3 lines

Do you have another idea how to achieve what I want? I was thinking of throttle the I/O to zero. It would be enough to do this for writes, reading doesn't hurt in my case.

Maybe add a block filter on top of the drained node, drain it when doing so, then queue all further requests with a CoQueue until "undrain". (It is then not quite to "drain" but to "halt" or "pause", though.)

To get the drain for free was why I was looking at this approach. If I read correctly if I keep using bdrv_drained_begin/end its too hacky to implement it in QMP?

Quoted 8 lines

I think so.

Quoted 1 line

If yes, would support adding it to qemu-io?

I'm under the impression that you are looking to a real use case, I don't think I like the idea. Also, accessing the image from other processes while QEMU is using it is strongly discouraged, and there is the coming image locking mechanism to prevent this from happening. Why is the blockdev-snapshot command not enough?

blockdev-snapshot is enough, but I still fear the case there is suddenly too much I/O for the live-commit. And that the whole snapshot / commit code is more senstive than just stopping I/O for a second or two.

do you have a pointer to the image locking mechanism?

Quoted 53 lines

Am 15.05.2017 um 14:52 schrieb Fam Zheng:

Quoted 52 lines · Peter Lieven

Quoted 40 lines

Am 15.05.2017 um 14:28 schrieb Fam Zheng:

Quoted 39 lines · Peter Lieven

Quoted 14 lines

Am 15.05.2017 um 13:53 schrieb Fam Zheng:

Quoted 13 lines · Peter Lieven

Quoted 12 lines

Am 15.05.2017 um 12:50 schrieb Fam Zheng:

Quoted 11 lines · Peter Lieven

Quoted 10 lines

Hi Block developers,

I would like to add a feature to Qemu to drain all traffic from a block so that I can take external snaphosts without the risk to that in the middle of a write operation. Its meant for cases where where QGA freeze/thaw is not available.

For me its enough to have this through qemu-io, but Kevin asked me to check if its not worth to have a stable API for it and present it via QMP/HMP.

What are your thoughts?

Quoted 8 lines

Quoted 6 lines

Quoted 4 lines

Quoted 3 lines

For debugging purpose or a "hacky" usage where you know what you are doing, it may be fine to have this. The only issue is it should be a separate flag, like BlockJob.user_paused.

How can I add, remove such a flag?

Like bs->user_drained. Set it in "drain" command, then increment bs->quiesce_counter if toggled; vise versa.

Ah okay. You wouldn't use bdrv_drained_begin/end? Because in these functions the counter is incremented already.

You're right, calling bdrv_drained_begin() is better.

Quoted 11 lines

Quoted 9 lines

Quoted 6 lines

Quoted 3 lines

What happens from guest perspective? In the case of virtio, the request queue is not handled and -ETIMEDOUT may happen. With IDE, I/O commands are still handled, the command is not effective (or rather the implementation is not complete).

That it only works with virtio is fine. However, the thing it does not work correctly apply then also to all other users of the drained_begin/end functions, right? As for the timeout I only plan to drain the device for about 1 second.

It didn't matter because for IDE, the invariant (staying quiesced as long as necessary) is already ensured by BQL. Virtio is different because it supports ioeventfd and data plane.

Okay understood. So my use of bdrv_drained_begin/end is more an abuse of these functions?

Sort of. But it's not unreasonable to "extend" bdrv_drained_begin/end to cover IDE, I just haven't thought about "how".

Quoted 8 lines

Quoted 6 lines

Quoted 3 lines

Do you have another idea how to achieve what I want? I was thinking of throttle the I/O to zero. It would be enough to do this for writes, reading doesn't hurt in my case.

Maybe add a block filter on top of the drained node, drain it when doing so, then queue all further requests with a CoQueue until "undrain". (It is then not quite to "drain" but to "halt" or "pause", though.)

To get the drain for free was why I was looking at this approach. If I read correctly if I keep using bdrv_drained_begin/end its too hacky to implement it in QMP?

I think so.

Quoted 1 line

If yes, would support adding it to qemu-io?

Quoted 8 lines

Quoted 5 lines

I'm under the impression that you are looking to a real use case, I don't think I like the idea. Also, accessing the image from other processes while QEMU is using it is strongly discouraged, and there is the coming image locking mechanism to prevent this from happening. Why is the blockdev-snapshot command not enough?

blockdev-snapshot is enough, but I still fear the case there is suddenly too much I/O for the live-commit. And that the whole snapshot / commit code is more senstive than just stopping I/O for a second or two.

In this case, the image fleecing approach may be what you need. It creates a temporary point in time snapshot which is lightweight and disposable. Something like:

https://lists.gnu.org/archive/html/qemu-devel/2014-03/msg01359.html

(Ccing John who may have more up-to-date pointers)

Quoted 2 lines

do you have a pointer to the image locking mechanism?

It hit qemu.git master just a moment ago. See raw_check_perm.

Quoted 2 lines

Quoted 1 line

do you have a pointer to the image locking mechanism?

It hit qemu.git master just a moment ago. See raw_check_perm.

which master are you looking at?

$ git fetch upstream
$ git log upstream/master --oneline
3a87606 Merge tag 'tracing-pull-request' into staging
b54933e Merge tag 'block-pull-request' into staging
3753e25 Merge remote-tracking branch 'kwolf/tags/for-upstream' into staging
5651743 trace: add sanity check
321d1db aio: add missing aio_notify() to aio_enable_external()
ee29d6a block: Simplify BDRV_BLOCK_RAW recursion
33c53c5 coroutine: remove GThread implementation
ecc1f5a maintainers: Add myself as linux-user reviewer
d541e20 Merge remote-tracking branch 'mreitz/tags/pull-block-2017-05-11' into queue-block
8dd30c8 MAINTAINERS: Add qemu-progress to the block layer
d2cb36a qcow2: Discard/zero clusters by byte count
f10ee13 qcow2: Assert that cluster operations are aligned

fbaa6bb qcow2: Optimize write zero of unaligned tail cluster
e249d51 iotests: Add test 179 to cover write zeroes with unmap
d9ca221 iotests: Improve _filter_qemu_img_map
06cc5e2 qcow2: Optimize zero_single_l2() to minimize L2 churn
fdfab37 qcow2: Make distinction between zero cluster types obvious
3ef9521 qcow2: Name typedef for cluster type
4341df8 qcow2: Correctly report status of preallocated zero clusters
4c41cb4 block: Update comments on BDRV_BLOCK_* meanings

Quoted 45 lines

Am 15.05.2017 um 14:28 schrieb Fam Zheng:

Quoted 44 lines · Peter Lieven

Quoted 14 lines

Am 15.05.2017 um 13:53 schrieb Fam Zheng:

Quoted 13 lines · Peter Lieven

Quoted 12 lines

Am 15.05.2017 um 12:50 schrieb Fam Zheng:

Quoted 11 lines · Peter Lieven

Quoted 10 lines

Hi Block developers,

I would like to add a feature to Qemu to drain all traffic from a block so that I can take external snaphosts without the risk to that in the middle of a write operation. Its meant for cases where where QGA freeze/thaw is not available.

For me its enough to have this through qemu-io, but Kevin asked me to check if its not worth to have a stable API for it and present it via QMP/HMP.

What are your thoughts?

Quoted 8 lines

Quoted 6 lines

Quoted 4 lines

Quoted 3 lines

For debugging purpose or a "hacky" usage where you know what you are doing, it may be fine to have this. The only issue is it should be a separate flag, like BlockJob.user_paused.

How can I add, remove such a flag?

Like bs->user_drained. Set it in "drain" command, then increment bs->quiesce_counter if toggled; vise versa.

Ah okay. You wouldn't use bdrv_drained_begin/end? Because in these functions the counter is incremented already.

You're right, calling bdrv_drained_begin() is better.

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What happens from guest perspective? In the case of virtio, the request queue is not handled and -ETIMEDOUT may happen. With IDE, I/O commands are still handled, the command is not effective (or rather the implementation is not complete).

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That it only works with virtio is fine. However, the thing it does not work correctly apply then also to all other users of the drained_begin/end functions, right? As for the timeout I only plan to drain the device for about 1 second.

It didn't matter because for IDE, the invariant (staying quiesced as long as necessary) is already ensured by BQL. Virtio is different because it supports ioeventfd and data plane.

Okay understood. So my use of bdrv_drained_begin/end is more an abuse of these functions?

Sort of. But it's not unreasonable to "extend" bdrv_drained_begin/end to cover IDE, I just haven't thought about "how".

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Do you have another idea how to achieve what I want? I was thinking of throttle the I/O to zero. It would be enough to do this for writes, reading doesn't hurt in my case.

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Maybe add a block filter on top of the drained node, drain it when doing so, then queue all further requests with a CoQueue until "undrain". (It is then not quite to "drain" but to "halt" or "pause", though.)

To get the drain for free was why I was looking at this approach. If I read correctly if I keep using bdrv_drained_begin/end its too hacky to implement it in QMP?

I think so.

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If yes, would support adding it to qemu-io?

I'm under the impression that you are looking to a real use case, I don't think I like the idea. Also, accessing the image from other processes while QEMU is using it is strongly discouraged, and there is the coming image locking mechanism to prevent this from happening.

Am 15.05.2017 um 14:52 hat Fam Zheng geschrieben:

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Hi Block developers,

I would like to add a feature to Qemu to drain all traffic from a block so that I can take external snaphosts without the risk to that in the middle of a write operation. Its meant for cases where where QGA freeze/thaw is not available.

For me its enough to have this through qemu-io, but Kevin asked me to check if its not worth to have a stable API for it and present it via QMP/HMP.

What are your thoughts?

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For debugging purpose or a "hacky" usage where you know what you are doing, it may be fine to have this. The only issue is it should be a separate flag, like BlockJob.user_paused.

How can I add, remove such a flag?

Like bs->user_drained. Set it in "drain" command, then increment bs->quiesce_counter if toggled; vise versa.

Ah okay. You wouldn't use bdrv_drained_begin/end? Because in these functions the counter is incremented already.

You're right, calling bdrv_drained_begin() is better.

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What happens from guest perspective? In the case of virtio, the request queue is not handled and -ETIMEDOUT may happen. With IDE, I/O commands are still handled, the command is not effective (or rather the implementation is not complete).

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That it only works with virtio is fine. However, the thing it does not work correctly apply then also to all other users of the drained_begin/end functions, right? As for the timeout I only plan to drain the device for about 1 second.

It didn't matter because for IDE, the invariant (staying quiesced as long as necessary) is already ensured by BQL. Virtio is different because it supports ioeventfd and data plane.

Okay understood. So my use of bdrv_drained_begin/end is more an abuse of these functions?

Sort of. But it's not unreasonable to "extend" bdrv_drained_begin/end to cover IDE, I just haven't thought about "how".

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Do you have another idea how to achieve what I want? I was thinking of throttle the I/O to zero. It would be enough to do this for writes, reading doesn't hurt in my case.

Maybe add a block filter on top of the drained node, drain it when doing so, then queue all further requests with a CoQueue until "undrain". (It is then not quite to "drain" but to "halt" or "pause", though.)

To get the drain for free was why I was looking at this approach. If I read correctly if I keep using bdrv_drained_begin/end its too hacky to implement it in QMP?

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I think so.

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If yes, would support adding it to qemu-io?

I'm under the impression that you are looking to a real use case, I don't think I like the idea. Also, accessing the image from other processes while QEMU is using it is strongly discouraged, and there is the coming image locking mechanism to prevent this from happening.

Thinking a bit more about this, it looks to me as if what we really want is inactivating the image. Should we add an option to the 'stop' command (or introduce another command to be used on an already stopped VM) that inactivates all/some images? And then 'cont' regains control over the images, just like after migration.

Automatically stopping the VM while the snapshot is taken also makes it work with IDE and prevents guests running into timeouts, which makes it look much more like a proper solution to me.

But then, stop/cont would already achieve something very similar today (as 'stop' calls bdrv_drain_all(); just the locking part wouldn't be covered), so maybe there is a reason not to use it in your case, Peter?

I would like to add a feature to Qemu to drain all traffic from a block so that I can take external snaphosts without the risk to that in the middle of a write operation. Its meant for cases where where QGA freeze/thaw is not available.

For me its enough to have this through qemu-io, but Kevin asked me to check if its not worth to have a stable API for it and present it via QMP/HMP.

What are your thoughts?