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Re: [Xen-devel] [RFC PATCH] xentop: add support for qdisks

Hi Charles,

It's good to see that someone else is tackling this problem. We have a similar 
problem in XenServer; tapdisk3 is a user space backend (just like qemu-qdisk) 
and therefore libxenstat is unable to pick up any statistics from it. In that 
way, "xentop" also doesn't display any stats for VBDs.

We tackled this by exposing statistics from tapdisk3 via /dev/shm. Our tools 
then pick up data from there.

You can look at the current code as implemented in tapdisk3 here:

And for a tool that consumes it here:

We are in the process of redesigning this slightly as we wish to break down the 
stats between input and output channels. tapdisk3 takes requests from several 
sources (e.g. blktap2, blkfront, nbd) and uses libaio to submit requests to 
different images. Currently, our code only consider requests coming from 
blkfront (or any other guest pv driver).

After we are finished, we intended to fix libxenstat to understand our format 
and consequently fix the stats as displayed by xentop. If you are committed to 
work on this now, perhaps it's worth touching base and agreeing on the data 
format and interfaces to prevent duplicated work.

Let us know what you think.


> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
> bounces@xxxxxxxxxxxxx] On Behalf Of Charles Arnold
> Sent: 20 February 2015 22:04
> To: xen-devel
> Cc: Ian Campbell
> Subject: Re: [Xen-devel] [RFC PATCH] xentop: add support for qdisks
> >>> On 2/20/2015 at 11:10 AM, Charles Arnold wrote:
> > Now that Xen uses qdisks by default and qemu does not write out
> > statistics to sysfs this patch queries the QMP for disk statistics.
> Forget this patch. I assumed the name of the xenstore backend device (eg, 
> xvda,
> hda, etc) would be the same name as used by qemu and it is not.
> - Charles
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