xentop's disk I/O

[Ulrich Windl]

Hi!

Watching xentop, I have a question:
I know that the network operations aren't accounted in xentop when
running Qubes OS, but I'm wondering:
For domain-0 all of the disk I/O (VBD_RD, VBD_WR, VBD_RSECT, VBD_WSECT)
also seem to be zero, and for sys-net-dm all the writes seem to be zero.
Is it because sys-net is a "HVM"?

Regards,
Ulrich Windl

[Jin-oh Kang]

On Sun, Nov 6, 2022, 9:29 AM Ulrich Windl <Ulrich...@rz.uni-regensburg.de> wrote:

Hi!

Watching xentop, I have a question:
I know that the network operations aren't accounted in xentop when
running Qubes OS, but I'm wondering:
For domain-0 all of the disk I/O (VBD_RD, VBD_WR, VBD_RSECT, VBD_WSECT)
also seem to be zero,

This is because dom0 does not use paravirtualized disk drivers. Specifically, no Xen block backend device in dom0 is connected to a block frontend device in dom0, since the latter does not exist. Instead, dom0 accesses disks directly like a bare-metal OS.

If you want disk I/O statistics for dom0, you want to use something like iotop.

and for sys-net-dm all the writes seem to be zero.

DM stands for "device model," a main role of which is to emulate the physical (i.e. non-virtualized) hardware for HVM guests that do not have the paravirtualized device drivers installed. If sys-net has all the necessary paravirtualized drivers (which almost always is the case), sys-net does not need the DM to access the disk. This is why sys-net-dm does not access the disk after boot.

Is it because sys-net is a "HVM"?

No. You'll observe non-zero disk I/O numbers in the DM when you boot e.g. a Windows HVM without Xen paravirtualized disk drivers.

Regards,
Ulrich Windl

[Ulrich Windl]

Thanks for explaining!

Ulrich

[Manuel Amador (Rudd-O)]

I maintain a Xen Prometheus exporter.  Here is what I know:

You will not see device writes or reads in the dm stubs because no process in them is reading or writing from disks.

The Prometheus exporter is awesome, BTW.  You can get system statistics and ingest them into Prometheus for system profiling and tuning.  I don't know how I lived using xentop before I began maintaining the exporter.  Watching Grafana is a far more satisfying way to explore data.