On Thu, 2020-01-16 at 17:03 +0800, Ming Lei wrote: > On Thu, Jan 16, 2020 at 12:06:02PM +0800, Yufen Yu wrote: > > Hi, all > > > > Shared tags is introduced to maintains a notion of fairness between > > active users. This may be good for nvme with multiple namespace to > > avoid starving some users. Right? > > Actually nvme namespace is LUN of scsi world. > > Shared tags isn't for maintaining fairness, it is just natural sw > implementation of scsi host's tags, since every scsi host shares > tags among all LUNs. If the SCSI host supports real MQ, the tags > is hw-queue wide, otherwise it is host wide. >From the standards point of view, this statement is incorrect, it's the ITLQ that identifies the Nexus, so the same tag *may* be in use for different commands on different LUNS according to standards. However, some device drivers choose to use ITQ as the identifier because it fits with the ITLQ standard and they can use Q as a per target mailbox identifier. I don't think this affects the analysis, but I just didn't want people to think it was always true. We have some older drivers that will allocate the same tag for different LUNS and this was incredibly prevalent in the SPI world where the tag was only bits. James
