Re: [Qemu-devel] [PATCH V3 2/2] Add -f option to qemu-nbd

[Chunyan Liu]

2011/11/24 Stefan Hajnoczi <address@hidden>

On Thu, Nov 24, 2011 at 3:38 AM, Chunyan Liu <address@hidden> wrote:
>
>
> 2011/11/23 Stefan Hajnoczi <address@hidden>
>>
>> On Wed, Nov 23, 2011 at 10:14 AM, Chunyan Liu <address@hidden> wrote:
>> > V3:
>> > Remove file lock in main().
>> > Try to find new free nbd device and connect to it if connecting to the
>> > first
>> > first found free nbd device failed.
>> >
>> > Signed-off-by: Chunyan Liu <address@hidden>
>> > ---
>> >  qemu-nbd.c |   80
>> > +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>> >  1 files changed, 79 insertions(+), 1 deletions(-)
>>
>> I not seeing the part where you adjusted the ioctl order.
>>
>> The /proc/partitions scanning is unnecessary since we can just loop
>> over /dev/ndb%d and try to initialize.  If a device is in use then
>> init will fail and we need to try the next one.  If a device is free
>> we can continue with normal operation.  I guess I'm saying that once
>> you fix the ioctl order then there's no need for another mechanism to
>> test whether or not a device is in use.
>
> The way of scanning /proc/partitions to find an unused nbd device first can
> borrow the code for "qemu-nbd -c" to do the left things. .
> The way of loop over /dev/nbd%d and try to initialize, from the first
> thought, needs do all things in the loop, including handling -v, nbd_init,
> nbd_client, etc. That part of code is quite similar to "qemu-nbd -c". I
> don't know if that is better?

This might be a chance to refactor the code slightly, that would also
avoid you having to introduce a goto retry.

About detail implementation of the loop over /dev/nbd%d way, have done some coding and testing. I'm afraid the reorganization work is not very slight, not sure how do you think.
First, through nbd_init() success or fail to check if the device is in use, at least nbd_init() and operations before nbd_init() but device relative should be in the loop. Usually, there are two ways to be considered:
1. Try to divide current "qemu-nbd -c" processing code into two parts ( nbd_init() and before, and operations after nbd_init()), 1st part put into the loop, 2nd part outside loop. This way is hard to achieve. nbd_init() should be in client thread, but the loop is outside that thread, it's not proper to drag out nbd_init() from client thread and put it into the loop.
2. Put all "qemu-nbd -c" processing code (device relative) in the loop, directly check the "qemu-nbd -c /dev/nbd%d disk.img" result, if fail, try next nbd device. In this way, will adjust current code order and extract device related codes to a new function so that both "qemu-nbd -c" and "qemu-nbd -f" can use.(some initialization work and cleanup work to be cared)

A draft is in attachment. qemu-nbd -f is working, since I'm not sure such change is acceptable or not, not extract code in the loop into a function yet. 

 

Stefan

test_find.patch
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