Re: [virtio] Re: [virtio-dev] Re: [virtio-comment] Problems with VIRTIO-4 and writeback only disks

[Rusty Russell <rusty@au1.ibm.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:
> Il 01/10/2013 12:13, Rusty Russell ha scritto:
>>>> +Note that buggy legacy devices are common, which always operate in
>>>> >> +write back mode even if VIRTIO_BLK_F_WCE isn't accepted, or writeback
>>>> >> +is turned off.
>>> >
>>> > I would remove this paragraph.  You found bugs, true, but it's not that
>>> > the affected versions of QEMU are common---they are neither in Debian
>>> > nor in RHEL/CentOS, and frankly using anything else in production would
>>> > be crazy (also if you do I hope you'll update fast, and those hosts will
>>> > not be "common" anymore by the time standardization is finished).
>> I was also thinking of this:
>> 
>> [30] Until version 1.1, QEMU remained in writeback mode even after a
>>      guest announced lack of support for VIRTIO_BLK_F_FLUSH.
>
> I would make this even stronger, like "It is undefined whether a disk
> operates in writeback or writethrough mode after a guest announces lack
> of support for VIRTIO_BLK_F_FLUSH".

At this point it's documenting historical behavior, rather than
proscribing it.  How about we simply note:

Some older legacy devices did not operate in writethrough mode even
after a guest announced lack of support for VIRTIO_BLK_F_FLUSH.

Cheers,
Rusty.