Hi Ming, Ming Lei <ming.lei@xxxxxxxxxx> 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? Best regards, Andreas Hindborg
