[SLOF] [PATCH 4/4] Provide "write" function in the disk-label package

[Thomas Huth]

On 24.11.2016 13:42, Alexey Kardashevskiy wrote:
> On 24/11/16 18:08, Thomas Huth wrote:
>> On 24.11.2016 08:04, Nikunj A Dadhania wrote:
>>> Alexey Kardashevskiy <aik at ozlabs.ru> writes:
>>>
>>>> On 16/11/16 00:02, Thomas Huth wrote:
>>>>> As with the "read" function, the disk-label package should
>>>>> forward the "write" function to its parent.
>>>>>
>>>>> Reviewed-by: Nikunj A Dadhania <nikunj at linux.vnet.ibm.com>
>>>>> Signed-off-by: Thomas Huth <thuth at redhat.com>
>>>>
>>>> After this patch, I cannot boot a guest at all:
>>>
>>> Works for me with the below command-line, am I missing something?
>>
>> Did you try to use a "grub-reboot" command, so that grub also tries to
>> write to the disk?
> 
> Where/why would I use this "grub-reboot"? :) I just tried a disk image with
> 2 kernels to see if grub writes/reads the last choice.

It's a Linux command, so you've got to boot to the Linux shell prompt
first. ... and actually, it's called "grub2-reboot" on my system, sorry
for the confusion.

>>>> qemu-system-ppc64: Incorrect order for descriptors
>>>>
>>>>
>>>> This is my cmdline (basically, virtio-scsi):
>>
>> Sorry, I did not check virtio-scsi, only vscsi and virtio-blk ... I'll
>> have a look ...
> 
> I am missing the point of virtio-blk and (especially) ibmvscsi, is there
> any case when either is better than virtio-scsi?

I'm lazy, so when I fire up a guest from the command line, I normally
only type "-hda mydisk.qcow2" instead of the whole -driver & -device
dance. But with -hda, you only get a vscsi image, so this was where I
looked at first.
Then, after that, I thought I'd cover all SCSI disks, since my patch
only affected the generic SCSI code, so I just had another look at
virtio-block.

But to answer your question in a more profund way:
- Apart from the easier command line, vscsi has the only advantage that
it might also work with very, very old guests that do not have drivers
for virtio-block or virtio-scsi yet.
- virtio-block might be a tiny, tiny little bit faster than virtio-scsi
since you can skip the SCSI emulation layer in this case
- AFAIK virtio-scsi has more features than virtio-block.

 Thomas