Re: the >1Tb block issue

[Avi Kivity <avi@redhat.com>]

On 05/18/2010 09:03 PM, Michael Tokarev wrote:
> 18.05.2010 21:38, Avi Kivity wrote:
>
>>> [Qemu-devel] [PATCH +stable] block: don't attempt to merge overlapping
>>> requests
>>>
>>> quick tests shows it works correctly so far.
>>> At least it went further than before, not
>>> stopping at the "usual" sector 3145727872.
>>>
>>> Hmm. Ide has no queue, hence no mergeing,
>>> that's why it does not occur with ide,
>>> right? :)
>>
>> Yes.
>
> I tried multiple times to reproduce it with if=scsi
> (queue_depth=16 for the sym53c8xx driver).  I can't.
> JFYI.. ;)

Merging needs explicit support in the block device emulation, which scsi 
lacks.

>
>>> Interesting...
>>
>> Yes. Why would Linux post overlapping requests? makes 0xffffffff00000000
>> sense.
>
> It's mkfs.

mkfs simply writes to the block device, even if it does issue 
overlapping writes, Linux shouldn't.  Either the writes contain the same 
content in the overlapping section, in which case it's redundant, or 
they don't, and we have data corruption in the making.

> Not sure why, but yes, maybe it's a guest
> bug after all. 

It's a host bug for sure, with a potential for a guest bug.

> Note that I'm running 64bit kernel on
> the guest (2.6.32.9-amd64).
>
> Note also that it's not as on the original bugreport -
> there, the sector# is apparently different:
> http://sourceforge.net/tracker/?func=detail&aid=2933400&group_id=180599&atid=893831

I don't think it's related to a particular sector number.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.