Re: [Qemu-discuss] Understanding QEMU Cache Modes and Emulated Disk Write Cache

[Fam Zheng]

On Tue, 08/16 12:39, Andrew Martin wrote:
> Hello,
> 
> I am running QEMU 2.5 on Ubuntu 16.04. My guests are a mixture of Linux 
> (Ubuntu
> 12.04 and newer with ext4) and Windows. The guest qcow2 images are stored in 
> a 
> ZFS filesystem with sync=standard (on top of a mirrored vdev of SSDs) on the 
> host
> Ubuntu 16.04 OS. After having read through documentation and some previous 
> threads on this list, I am still somewhat confused about caching.
> 
> First, my understanding is that if I strace the qemu process and see it
> regularly issuing fdatasync(2), this means WCE is enabled and it is safe to 
> use
> cache=writeback, correct?
> 
> I see fdatasync(2) coming through about every 6 seconds, so I believe the 
> guest
> is properly flushing to disk. Now in this setup, my understanding is there 
> are 4
> caches and permanent storage:
> * guest page cache
> * guest disk write cache (virtio)
> * host page cache
> * host disk write cache (SSD)
> * permanent storage
> 
> I understand that dirty pages get queued up in the guest page cache, and that
> when an ext4 barrier is hit an fdatasync(2) call causes them to get flushed to
> disk. At this point, does fdatasync(2) force the data into the guest disk 
> write
> cache, the host page cache, the host disk write cache, or permanent storage?
> Does this vary depending on if the virtio disk is set to cache=writeback or
> cache=writethrough? Can you explain how this works for both cases?

With cache=writeback, a guest WRITE will put data to host page cache; a guest
FLUSH will flush to permanent storage.

With cache=writethrough, a guest WRITE will put data to host page cache, and
then flushed to permanent storage with a host side fdatasync issued by QEMU. In
this case the guest sees no write cache on the virtual device.

> 
> I am confused about the difference between the guest disk write cache and the
> host page cache. Aren't these effectively the same (in both cases storing the
> dirty data in the host's RAM)? Is there a difference between how one gets 

Yes they are the same thing.

Fam

> flushed and how the other gets flushed? Or do writes get written to the guest 
> disk write cache first, then the host page cache?

> 
> Thanks for the help!
> 
> Andrew
>