Re: [Qemu-devel] [PATCH] block: allow write-threshold on device name

[Eric Blake <eblake@redhat.com>]

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On 06/06/2015 07:38 PM, Eric Blake wrote:
> Commit e2462113 allowed the ability to fire an event if a BDS
> node exceeds a threshold during a write, but limited the option
> to only work on node names.  For convenience, expand this to
> allow a device name as a way to set the threshold on the BDS
> at the active layer of the device.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
>  block/write-threshold.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 

> @@ -110,9 +110,8 @@ void qmp_block_set_write_threshold(const char *node_name,
>      BlockDriverState *bs;
>      AioContext *aio_context;
> 
> -    bs = bdrv_find_node(node_name);
> +    bs = bdrv_lookup_bs(node_name, node_name, errp);

Hmm, I'm not quite sure this does what I want.  If I understand
correctly, when you open a qcow2 image, you get the following
query-block layout (abbreviated):

            "inserted": {
...
                "image": {
...
                    "backing-filename-format": "qcow2",
                    "virtual-size": 12884901888,
                    "filename": "/dev/sda6",
                    "cluster-size": 65536,
                    "format": "qcow2",
                    "actual-size": 0,
...
                "drv": "qcow2",
                "iops": 0,
                "bps_wr": 0,
                "write_threshold": 0,
...
                "file": "/dev/sda6",


That is, the only write_threshold reported is that of the active layer
BDS (write_threshold of other BDS is reported through
query-named-block-nodes, but only if those BDS have a name), but
query-block is not listing any secondary BDS details.

Meanwhile, here is the corresponding query-blockstats layout:

            "device": "drive-virtio-disk0",
            "parent": {
                "stats": {
                    "flush_total_time_ns": 0,
                    "wr_highest_offset": 72482304,
...
            "stats": {
                "flush_total_time_ns": 728455560,
                "wr_highest_offset": 9129332224,

which DOES show the BDS chain; in particular, each qcow2 file has two
BDS (one for the protocol, and the other ('parent') for the actual file).

The statistic I'm interested in is the allocation of the block device
(the host offset, aka wr_highest_offset 72482304 above), and NOT the
usage pattern of the guest (the qcow2 protocol, wr_highest_offset
9129332224).  But bdrv_lookup_bs() finds the qcow2 protocol layer,
rather than the intended backing file layer; likewise, query-block is
only reporting write_threshold for the protocol layer.

I'm wondering if, when a device name is given rather than a node name,
it is safe to blindly follow the active layer down to its lowest member
(or error out if there are more than one lower members, as in quorum),
as that is the statistic that libvirt and upper layers really want ("am
I about to exceed the allocation of my underlying storage?").  Likewise,
on reporting, it is more useful to know the threshold of the backing
layer if the qcow2 protocol layer does not have a threshold.  I'm
playing with that idea before submitting a v2.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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