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

[Michael Tokarev <mjt@tls.msk.ru>]

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
> 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.

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.

Is it possible?

Thanks,

/mjt