Re: [PATCHSET v3][RFC] Make background writeback not suck

[Jens Axboe]

On 04/01/2016 12:27 AM, Dave Chinner wrote:
> On Thu, Mar 31, 2016 at 09:25:33PM -0600, Jens Axboe wrote:
> > On 03/31/2016 06:46 PM, Dave Chinner wrote:
> > > > > virtio in guest, XFS direct IO -> no-op -> scsi in host.
> > > >
> > > > That has write back caching enabled on the guest, correct?
> > >
> > > No. It uses virtio,cache=none (that's the "XFS Direct IO" bit above).
> > > Sorry for not being clear about that.
> >
> > That's fine, it's one less worry if that's not the case. So if you
> > cat the 'write_cache' file in the virtioblk sysfs block queue/
> > directory, it says 'write through'? Just want to confirm that we got
> > that propagated correctly.
>
> No such file. But I did find:
>
> $ cat /sys/block/vdc/cache_type
> write back
>
> Which is what I'd expect it to safe given the man page description
> of cache=none:
>
> 	Note that this is considered a writeback mode and the guest
> 	OS must handle the disk write cache correctly in order to
> 	avoid data corruption on host crashes.
>
> To make it say "write through" I need to use cache=directsync, but
> I have no need for such integrity guarantees on a volatile test
> device...

I wasn't as concerned about the integrity side, more if it's flagged as 
write back then we induce further throttling. But I'll see if I can get 
your test case reproduced, then I don't see why it can't get fixed. I'm 
off all of next week though, so probably won't be until the week after...

--
Jens Axboe