Re: [Qemu-devel] [V2 2/2]Qemu: Add commands "hostcache_set" and "hostcache_get"

[Kevin Wolf <kwolf@redhat.com>]

Am 23.05.2011 12:00, schrieb Stefan Hajnoczi:
> On Mon, May 23, 2011 at 8:04 AM, Supriya Kannery <supriyak@in.ibm.com> wrote:
>> On 05/20/2011 01:50 PM, Stefan Hajnoczi wrote:
>>>
>>> On Thu, May 19, 2011 at 10:38:03PM +0530, Supriya Kannery wrote:
>>>>
>>>> Monitor commands "hostcache_set" and "hostcache_get" added for dynamic
>>>> host cache change and display of host cache setting respectively.
>>>
>>> A generic command for changing block device options would be nice,
>>> althought I don't see other options where it makes sense to change them
>>> at runtime.
>>>
>>> The alternative would be:
>>>
>>> block_set hostcache on
>>>
>>> "block_set", {"device": "ide1-cd0", "name": "hostcache", "enable": true}
>>>
>>> The hostcache_get information would be part of query-block output:
>>>          {
>>>             "device":"ide0-hd0",
>>>             "locked":false,
>>>             "removable":false,
>>>             "inserted":{
>>>                "ro":false,
>>>                "drv":"qcow2",
>>>                "encrypted":false,
>>>                "file":"disks/test.img"
>>>               "hostcache":true,
>>>             },
>>>             "type":"hd"
>>>          },
>>>
>>> This approach is extensible if more options need to be exposed.
>>
>> Sure, I will resubmit this patchset, after making this feature more generic.
>> Can you pls help finding atleast one or two options (other than hostcache)
>> which can be changed dynamically. This will help me evaluate the generic
>> approach.
> 
> Hang on, let's see if we can get agreement from Kevin and others
> before taking this approach.  Like I said, I don't see other options
> that should be changed at runtime.

Things like enabling copy on read could fit here.

Generally I'm in favour of having a generic command. We just need to pay
attention not to include things that we don't want to maintain long
term, i.e. just putting the current cache=... parameter into the
argument isn't going to work. Maybe two booleans 'o_direct' and
'ignore_flushes' is what we want to have. The same structure should be
used for blkdev_add then, even though it will allow some more options.

I'm also not completely sure how you would enable cache=writethrough
from the command line in a fully converted world. Would this be one of
the arguments that must be specified on BlockDriverState creation and
can't be changed later, and the device will pick it up from there? Or is
it a qdev property and somehow makes it way to the block layer?

Kevin