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.