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[Qemu-devel] Is cache=writeback safe yet?
From: |
Virtbie |
Subject: |
[Qemu-devel] Is cache=writeback safe yet? |
Date: |
Mon, 20 Feb 2012 15:18:43 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:8.0) Gecko/20111124 Thunderbird/8.0 |
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.
- [Qemu-devel] Is cache=writeback safe yet?,
Virtbie <=