Re: [qemu-web PATCH v2] Add virtio-blk and virtio-scsi configuration post

[Stefan Hajnoczi]

On Wed, Jan 06, 2021 at 03:04:20PM +0100, Thomas Huth wrote:
> On 05/01/2021 11.49, Stefan Hajnoczi wrote:
> > The second post in the storage series covers virtio-blk and virtio-scsi.
> > It compares the two and offers recommendations that users and tools
> > using QEMU can use as a starting point. Graphs are included comparing
> > the performance of various options.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> > v2:
> >   * Mention default storage controllers for non-x86 [danpb]
> >   * Remove statement about read-only ISO virtio-blk devices, both Rich
> >     Jones and Daniel Berrange commented about this at different times so
> >     I decided to drop it completely [danpb]
> >   * Clarify PCI slot limits and multi-function discussion for virtio-blk
> >     [danpb]
> >   * Mention multiple virtio-scsi devices for NUMA tuning and that
> >     emulated LUNs can be HDDs or CD-ROMs [danpb]
> > ---
> >   ...020-12-17-virtio-blk-scsi-configuration.md | 115 ++++++++++++++++++
> >   screenshots/2020-09-15-scsi-devices.svg       |   1 +
> >   screenshots/2020-09-15-virtio-blk-vs-scsi.svg |   1 +
> >   3 files changed, 117 insertions(+)
> >   create mode 100644 _posts/2020-12-17-virtio-blk-scsi-configuration.md
> >   create mode 100644 screenshots/2020-09-15-scsi-devices.svg
> >   create mode 100644 screenshots/2020-09-15-virtio-blk-vs-scsi.svg
> > 
> > diff --git a/_posts/2020-12-17-virtio-blk-scsi-configuration.md 
> > b/_posts/2020-12-17-virtio-blk-scsi-configuration.md
> > new file mode 100644
> > index 0000000..b4361f5
> > --- /dev/null
> > +++ b/_posts/2020-12-17-virtio-blk-scsi-configuration.md
> > @@ -0,0 +1,115 @@
> > +---
> > +layout: post
> > +title:  "Configuring virtio-blk and virtio-scsi Devices"
> > +date:   2021-01-05 07:00:00 +0000
> > +author: Stefan Hajnoczi and Sergio Lopez
> > +categories: [storage]
> > +---
> > +The [previous 
> > article](https://www.qemu.org/2020/09/14/qemu-storage-overview/)
> > +in this series introduced QEMU storage concepts. Now we move on to look at 
> > the
> > +two most popular emulated storage controllers for virtualization: 
> > virtio-blk
> > +and virtio-scsi.
> > +
> > +This post provides recommendations for configuring virtio-blk and 
> > virtio-scsi
> > +and how to choose between the two devices. The recommendations provide good
> > +performance in a wide range of use cases and are suitable as default 
> > settings
> > +in tools that use QEMU.
> > +
> > +## Virtio storage devices
> > +### Key points
> > +* Prefer virtio storage devices over other emulated storage controllers.
> > +* Use the latest virtio drivers.
> > +
> > +Virtio devices are recommended over other emulated storage controllers as 
> > they
> > +are generally the most performant and fully-featured storage controllers in
> > +QEMU.
> > +
> > +Unlike emulations of hardware storage controllers, virtio-blk and 
> > virtio-scsi
> > +are specifically designed and optimized for virtualization. The details of 
> > how
> > +they work are published for driver and device implementors in the [VIRTIO
> > +specification](https://docs.oasis-open.org/virtio/virtio/v1.1/virtio-v1.1.html).
> > +
> > +Virtio drivers are available for both Linux and Windows virtual machines.
> 
> Maybe mention "and other operating systems" as well? I assume virtio drivers
> are also available for the various BSDs etc.

Will fix.

> 
> > +Installing the latest version is recommended for the latest bug fixes and
> > +performance enhancements.
> > +
> > +If virtio drivers are not available, the AHCI (SATA) device is widely 
> > supported
> > +by modern x86 operating systems and can be used as a fallback. On non-x86
> > +guests the default storage controller can be used as a fallback.
> > +
> > +## Comparing virtio-blk and virtio-scsi
> > +### Key points
> > +* Prefer virtio-scsi for attaching more than 28 disks or for full SCSI 
> > support.
> 
> I'd rather say "lots of devices" here instead of "28", since 28 needs an
> explanation (which you give below).

It's unclear whether "lots" means 5, 10, 100, or 1000. I think we should
give a number to help readers understand the scale.

> 
> > +* Prefer virtio-blk in performance-critical use cases.
> 
> Maybe move the virtio-blk bullet above the virtio-scsi bullet, so that the
> two virtio-scsi bullets are listed next to each other? And it's also the
> order that you use below.

Will fix.

> 
> > +* With virtio-scsi, use scsi-block for SCSI passthrough and otherwise use 
> > scsi-hd.
> > +
> > +Two virtio storage controllers are available: virtio-blk and virtio-scsi.
> > +
> > +### virtio-blk
> > +The virtio-blk device presents a block device to the virtual machine. Each
> > +virtio-blk device appears as a disk inside the guest. virtio-blk was 
> > available
> > +before virtio-scsi and is the most widely deployed virtio storage 
> > controller.
> > +
> > +The virtio-blk device offers high performance thanks to a thin software 
> > stack
> > +and is therefore a good choice when performance is a priority.
> 
> I'd maybe add some more words here, saying that virtio-blk is really simple,
> just read and write, and that other features might not be supported (like
> ejecting CD devices). Maybe also add a comment that it took a long time to
> add an additional command like TRIM to the device?

I'll extend the next paragraph to also include devices with non-disk
functionality (like CD-ROMs). virtio-scsi is good for that.

> > +Applications that send SCSI commands are better served by the virtio-scsi
> > +device, which has full SCSI support. SCSI passthrough was removed from the
> > +Linux virtio-blk driver in v5.6 in favor of using virtio-scsi.
> 
> Since "passthrough" sounds like a passthrough of real hardware devices, I'd
> maybe rather say something like: The original virtio-blk devices also
> included the possibility to handle SCSI commands, but this features was
> removed from the Linux driver in v5.6 in favor of using virtio-scsi (and is
> also not available for modern virtio-1.0 devices anymore).

Guest SCSI commands were passed through to the physical LUN but what you
described sounds more like emulating SCSI commands, which is not what
the feature did.

I'm confused now. Should I keep the text or can you rephrase what you
mean?

> > +## Conclusion
> > +The virtio-blk and virtio-scsi offer a choice between a single block 
> > device and
> > +a full-fledged SCSI Host Bus Adapter. Virtualized guests typically use one 
> > or
> > +both of them depending on functional and performance requirements. This 
> > post
> > +compared the two and offered recommendations on how to choose between them.
> > +
> > +The next post in this series will discuss the iothreads feature that both
> > +virtio-blk and virtio-scsi support for increased performance.
> 
> Do you maybe also want to give examples for the QEMU command line showing
> how virtio-blk and virtio-scsi can be used there?

Not in this post. In the upcoming NUMA tuning discussion I think syntax
is really helpful though.

Stefan

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