Re: [Libvir] Re; virDomainBlockStats
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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