[Qemu-devel] Is cache=writeback safe yet?

[Virtbie <virtbie@shiftmail.org>]

Dear qemuers,
thanks for your exellent software.

I would like to use cache=writeback, but I still can't understand if 
this is safe or not in case of power loss.
In particular with virtio-blk on bare LVM device.

Qemu manpage still says cache=writeback isn't safe, but I seem to find 
some discordant information reading around.

Is the WCE+volatile flag exposed to the guest already? I am logging into 
a 2.6.38 guest and trying to find an indicator of disk cache in 
/sys/block/vda/device but I am not able to. There is an obscure very 
long "features" bitmask which I don't know what it shows. I think this 
mode is not safe until WBC is exposed, AFAIU, right?


Also please have a look at this:
https://events.linuxfoundation.org/slides/2011/linuxcon-japan/lcj2011_hajnoczi.pdf
page "Caching modes in Qemu"
In the table it is written that writeback has "Guest disk write cache = on".
Does that mean that the guest can commit to real platters by issuing a 
flush on the virtual device?
So the problem lies in the fact that no guest will ever spontaneously 
issue the flush because they don't see a wce=1?


There is another thing I don't understand:
I think I read somewhere that cache=none is safer than cache=writeback. 
Is that true? I think that both have a writeback cache, one is in the 
physical disks attached to the host, the other is in the host's page 
cache, so they should have about the same level of safety. What do you 
think?


Lastly, regarding the newly introduced cache=directsync. Do I understand 
correctly that it is supposed to be as safe as cache=writeback but also 
as slow as cache=writeback, for writes, and in addition it cannot use 
the host pagecache for reads, is that correct?


BTW I also wanted to say I very much like the simplification proposed by 
Anthony Liguori on 06/29/2011:

> Thinking twice about this, shouldn't we just move to a simplified model:
>
> -drive file=foo.img,cache=[on|off],hd0  -device 
> virtio-blk-pci,drive=hd0,wce=1

splitting the cache features in two (host side / guest side) would very 
much clarify what is happening in Qemu, for us ignorant users.


Thank you
Vb.