Re: [LSF/MM/BPF BoF]: extend UBLK to cover real storage hardware

[Andreas Hindborg <nmi@metaspace.dk>]

Ming Lei <ming.lei@redhat.com> writes:

> On Thu, Feb 16, 2023 at 10:44:02AM +0100, Andreas Hindborg wrote:
>> 
>> Hi Ming,
>> 
>> Ming Lei <ming.lei@redhat.com> writes:
>> 
>> > On Mon, Feb 13, 2023 at 02:13:59PM -0500, Stefan Hajnoczi wrote:
>> >> On Mon, Feb 13, 2023 at 11:47:31AM +0800, Ming Lei wrote:
>> >> > On Wed, Feb 08, 2023 at 07:17:10AM -0500, Stefan Hajnoczi wrote:
>> >> > > On Wed, Feb 08, 2023 at 10:12:19AM +0800, Ming Lei wrote:
>> >> > > > On Mon, Feb 06, 2023 at 03:27:09PM -0500, Stefan Hajnoczi wrote:
>> >> > > > > On Mon, Feb 06, 2023 at 11:00:27PM +0800, Ming Lei wrote:
>> >> > > > > > Hello,
>> >> > > > > > 
>> >> > > > > > So far UBLK is only used for implementing virtual block device from
>> >> > > > > > userspace, such as loop, nbd, qcow2, ...[1].
>> >> > > > > 
>> >> > > > > I won't be at LSF/MM so here are my thoughts:
>> >> > > > 
>> >> > > > Thanks for the thoughts, :-)
>> >> > > > 
>> >> > > > > 
>> >> > > > > > 
>> >> > > > > > It could be useful for UBLK to cover real storage hardware too:
>> >> > > > > > 
>> >> > > > > > - for fast prototype or performance evaluation
>> >> > > > > > 
>> >> > > > > > - some network storages are attached to host, such as iscsi and nvme-tcp,
>> >> > > > > > the current UBLK interface doesn't support such devices, since it needs
>> >> > > > > > all LUNs/Namespaces to share host resources(such as tag)
>> >> > > > > 
>> >> > > > > Can you explain this in more detail? It seems like an iSCSI or
>> >> > > > > NVMe-over-TCP initiator could be implemented as a ublk server today.
>> >> > > > > What am I missing?
>> >> > > > 
>> >> > > > The current ublk can't do that yet, because the interface doesn't
>> >> > > > support multiple ublk disks sharing single host, which is exactly
>> >> > > > the case of scsi and nvme.
>> >> > > 
>> >> > > Can you give an example that shows exactly where a problem is hit?
>> >> > > 
>> >> > > I took a quick look at the ublk source code and didn't spot a place
>> >> > > where it prevents a single ublk server process from handling multiple
>> >> > > devices.
>> >> > > 
>> >> > > Regarding "host resources(such as tag)", can the ublk server deal with
>> >> > > that in userspace? The Linux block layer doesn't have the concept of a
>> >> > > "host", that would come in at the SCSI/NVMe level that's implemented in
>> >> > > userspace.
>> >> > > 
>> >> > > I don't understand yet...
>> >> > 
>> >> > blk_mq_tag_set is embedded into driver host structure, and referred by queue
>> >> > via q->tag_set, both scsi and nvme allocates tag in host/queue wide,
>> >> > that said all LUNs/NSs share host/queue tags, current every ublk
>> >> > device is independent, and can't shard tags.
>> >> 
>> >> Does this actually prevent ublk servers with multiple ublk devices or is
>> >> it just sub-optimal?
>> >
>> > It is former, ublk can't support multiple devices which share single host
>> > because duplicated tag can be seen in host side, then io is failed.
>> >
>> 
>> I have trouble following this discussion. Why can we not handle multiple
>> block devices in a single ublk user space process?
>> 
>> From this conversation it seems that the limiting factor is allocation
>> of the tag set of the virtual device in the kernel? But as far as I can
>> tell, the tag sets are allocated per virtual block device in
>> `ublk_ctrl_add_dev()`?
>> 
>> It seems to me that a single ublk user space process shuld be able to
>> connect to multiple storage devices (for instance nvme-of) and then
>> create a ublk device for each namespace, all from a single ublk process.
>> 
>> Could you elaborate on why this is not possible?
>
> If the multiple storages devices are independent, the current ublk can
> handle them just fine.
>
> But if these storage devices(such as luns in iscsi, or NSs in nvme-tcp)
> share single host, and use host-wide tagset, the current interface can't
> work as expected, because tags is shared among all these devices. The
> current ublk interface needs to be extended for covering this case.

Thanks for clarifying, that is very helpful.

Follow up question: What would the implications be if one tried to
expose (through ublk) each nvme namespace of an nvme-of controller with
an independent tag set? What are the benefits of sharing a tagset across
all namespaces of a controller?

Best regards,
Andreas