[Qemu-devel] Question about block driver

[Qemu-devel] Question about block driver

[Wen Congyang]

Hi, all

Some drivers use bdrv_open, while the other dirvers use bdrv_file_open().
What is the difference between bdrv_open() and bdrv_file_open()?

Thanks
Wen Congyang

Re: [Qemu-devel] Question about block driver

[Kevin Wolf]

[Cc: qemu-block]

Am 30.04.2015 um 11:11 hat Wen Congyang geschrieben:
> Some drivers use bdrv_open, while the other dirvers use bdrv_file_open().
> What is the difference between bdrv_open() and bdrv_file_open()?

bdrv_file_open() is used by protocol drivers that don't need any other
driver to work (e.g. raw-posix), bdrv_open() is used by format drivers
that need a bs->file (e.g. raw or qcow2).

They used to have different parameters originally, but nowadays that's
the only difference.

Kevin

Re: [Qemu-devel] Question about block driver

[Wen Congyang]

On 04/30/2015 05:33 PM, Kevin Wolf wrote:
> [Cc: qemu-block]
> 
> Am 30.04.2015 um 11:11 hat Wen Congyang geschrieben:
>> Some drivers use bdrv_open, while the other dirvers use bdrv_file_open().
>> What is the difference between bdrv_open() and bdrv_file_open()?
> 
> bdrv_file_open() is used by protocol drivers that don't need any other
> driver to work (e.g. raw-posix), bdrv_open() is used by format drivers
> that need a bs->file (e.g. raw or qcow2).
> 
> They used to have different parameters originally, but nowadays that's
> the only difference.

So, if the driver wants to open bs->file itself, it should use bdrv_file_open(),
and the driver wants bs->file has already been opened before its open, it should
use bdrv_open(). Is it right?

Thanks
Wen Congyang

> 
> Kevin
> .
> 

Re: [Qemu-devel] Question about block driver

[Kevin Wolf]

Am 30.04.2015 um 11:43 hat Wen Congyang geschrieben:
> On 04/30/2015 05:33 PM, Kevin Wolf wrote:
> > [Cc: qemu-block]
> > 
> > Am 30.04.2015 um 11:11 hat Wen Congyang geschrieben:
> >> Some drivers use bdrv_open, while the other dirvers use bdrv_file_open().
> >> What is the difference between bdrv_open() and bdrv_file_open()?
> > 
> > bdrv_file_open() is used by protocol drivers that don't need any other
> > driver to work (e.g. raw-posix), bdrv_open() is used by format drivers
> > that need a bs->file (e.g. raw or qcow2).
> > 
> > They used to have different parameters originally, but nowadays that's
> > the only difference.
> 
> So, if the driver wants to open bs->file itself, it should use bdrv_file_open(),
> and the driver wants bs->file has already been opened before its open, it should
> use bdrv_open(). Is it right?

Yes, that's how it works, even though I don't think a driver exists that
opens bs->file by itself.

Kevin

Re: [Qemu-devel] Question about block driver

[Stefan Hajnoczi]

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On Thu, Apr 30, 2015 at 11:52:01AM +0200, Kevin Wolf wrote:
> Am 30.04.2015 um 11:43 hat Wen Congyang geschrieben:
> > On 04/30/2015 05:33 PM, Kevin Wolf wrote:
> > > [Cc: qemu-block]
> > > 
> > > Am 30.04.2015 um 11:11 hat Wen Congyang geschrieben:
> > >> Some drivers use bdrv_open, while the other dirvers use bdrv_file_open().
> > >> What is the difference between bdrv_open() and bdrv_file_open()?
> > > 
> > > bdrv_file_open() is used by protocol drivers that don't need any other
> > > driver to work (e.g. raw-posix), bdrv_open() is used by format drivers
> > > that need a bs->file (e.g. raw or qcow2).
> > > 
> > > They used to have different parameters originally, but nowadays that's
> > > the only difference.
> > 
> > So, if the driver wants to open bs->file itself, it should use bdrv_file_open(),
> > and the driver wants bs->file has already been opened before its open, it should
> > use bdrv_open(). Is it right?
> 
> Yes, that's how it works, even though I don't think a driver exists that
> opens bs->file by itself.

Some extra context about this statement: .bdrv_file_open() block drivers
(aka "protocols") operate directly on a POSIX file descriptor, a socket,
GlusterFS API, etc so they usually have no need for bs->file.

Stefan

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