Re: qcow2 images make scrub believe the filesystem is corrupted.

[Liu Bo <bo.li.liu@oracle.com>]

On Fri, Aug 18, 2017 at 06:23:18PM +0200, Goffredo Baroncelli wrote:
> On 08/18/2017 01:39 AM, Josef Bacik wrote:
> [...]
> > This is happening because the app (the guest OS in this case, we saw this a lot
> > with windows guests) is changing the pages while they are in flight.  We
> > calculate the checksum of the page before it's written, so if it changes while
> > in flight we'll end up with a csum mismatch.
> > 
> > To fix this change kvm to not use O_DIRECT or set NODATASUM on your qcow2 image.
> > You'll have to re-create the image because NODATASUM won't apply to the already
> > invalid checksums.  Thanks,
> 
> Hi Josef,
> 
> could you elaborate: do you are saying that using O_DIRECT is incompatible with DATASUM ?
>

They're compatible, but applications need to be careful.

O_DIRECT takes userspace page, and it works like
DIO write
  p = get_user_page();
  add p to bio
  #btrfs submits this bio
  calc_checksum(bio);
  submit_bio();

There's a chance that page p got changed between calc_checksum() and
submit_bio(), which then causes the mismatch.

For buffered IO, dirty page cache pages is synchronized with page
fault by page lock and page writeback bit.

thanks,
-liubo