Re: [Qemu-devel] block-commit & dropping privs

[Michael Tokarev]

01.04.2015 12:26, Michael Tokarev пишет:
> 30.03.2015 18:36, Kevin Wolf wrote:
>> Am 27.03.2015 um 18:12 hat Eric Blake geschrieben:
>>> On 03/27/2015 09:36 AM, Michael Tokarev wrote:
>>>> Wonder how to specify cache mode, or should I open these with proper
>>>> O_DIRECT/O_SYNC/whatever?  It looks like it's possible to change O_DIRECT
>>>> at runtime but not O_SYNC.
>>>>
>>>> And the more interesting question is how to do that from shell.
>>>
>>> Redirections only get you so far in shell; you may need a wrapper C
>>> program go get O_DIRECT and/or O_SYNC pre-set.  Then again, if you use
>>> QMP and pass over the Unix socket, you need a C program anyways.
>>
>> O_DIRECT can be set with fcntl(), so qemu takes care of that. O_SYNC
>> is completely unused on Linux these days, so that shouldn't be a problem

Do you mean it is unused in qemu or in kernel? :)

>> either. (Other platforms use it as a misguided attempt of approximating
>> O_DIRECT. We should really error out instead.)
>>
>> So if I'm not mistaken, just having one read-only and one read-write fd
>> should be enough for any configuration in practice.

Yes, O_DIRECT appears to work with fdsets.

> Probably yes.  Except the thing doesn't actually work. ;)
> 
> When flushing changes to the base image, that base image needs to be
> reopened.  So I should convince qemu that the base image of this qcow
> file is /dev/fdset/foo, not the one recorded in the header.

qemu-system-x86_64: -drive 
file=w7x64.qcow2,backing_file=/dev/fdset/2,if=none,id=dr,cache=none: \
 could not open disk image w7x64.qcow2: Block format 'qcow2' used by device 
'dr' \
 doesn't support the option 'backing_file'

hmm?..

> Is it possible?

Thanks,

/mjt
>