[Libvir] Re; virDomainBlockStats

[Daniel P. Berrange]

On Sun, Jan 27, 2008 at 02:53:46PM +0000, Gareth Bult wrote:
> Ok, question;
> 
> Does TAP:AIO work on networked filesystems .. my testing says not.

It does on Fedora / RHEL at least - it should work on any FS that
supports direct IO & async IO.

> XEN without a good underlying cluster filesystem is to say the 
> least "limited". If you can't use AIO in this sort of environment, 
> it also becomes limited .. and if libvirt requires this, it means 
> libvirt can't be used on large-scale roll-outs on (certain?) 
> network filesystems.

We need to fix libvirt so that stats work with file: based devices
too. We just need to figure out the logic required in order to
generate the correct device ID.

> Interestingly (looking at the Xen list re; performance problems) 
> I'm getting > 65Mb/sec on my DomU's .vs. 70Mb/sec on my host 
> nodes Dom0 ... and it's proving to be reliable atm .. 

You ought to be able to get within 5% of host performance when 
using either phy: (for real block devs) or tap:aio:. NB make
sure you do *not* use sparse files - fully allocate the file
if you want good performance. Sparse file have terrible performance
characteristics as blocks as allocated on-demand which causes
metadata syncs of the underlying host FS, which serializes all
I/O operations.

> Be interesting to know why Xen only documents "file" given 
> the critical nature / apparent flaws in the driver.

Xen documentation leeaves alot to be desired :-(

> Is there "no" way of forcing a loopback interface to flush itself?

Unfortunately not - its inherant in the impl of loop devices.

Dan.
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