Hello, [sorry for cross-posting, I think this is relevant to multiple communities.] I'm referring to the recent discussion about SCSI device identification for multipath-tools (https://listman.redhat.com/archives/dm-devel/2021-March/msg00332.html) As you all know, there are different designators to identify SCSI LUNs, and the specs don't mandate priorities for devices that support multiple designator types. There are various implementations for device identification, which use different priorities (summarized below). It's highly desirable to clean up this confusion and settle on a single instance and a unique priority order. I believe this instance should be the kernel. OTOH, changing device WWIDs is highly dangerous for productive systems. The WWID is prominently used in multipath-tools, but also in lots of other important places such as fstab, grub.cfg, dracut, etc. No doubt that we'll be stuck with the different algorithms for years, especially for LTS distributions. But perhaps we can figure out a long-term exit strategy? The kernel's preference for type 8 designators (see below) is in contrast with the established user space algorithms, which determine SCSI WWIDs on productive systems in practice. User space can try to adapt to the kernel logic, but it will necessarily be a slow and painful path if we want to avoid breaking user setups. In principle, I believe the kernel is "right" to prefer type 8. But because the "wwid" attribute isn't actually used for device identification today, changing the kernel logic would be less prone to regressions than changing user space, even if it violates the principle that the kernel's user space API must remain stable. Would it be an option to modify the kernel logic? If we can't, I think we should start with making the "wwid" attribute part of the udev rule logic, and letting distros configure whether the kernel logic or the traditional udev logic would be used. Please tell me your thoughts on this matter. Regards, Martin PS: Incomplete list of algorithms for SCSI designator priorities: The kernel ("wwid" sysfs attribute) prefers "SCSI name string" (type 8) designators over other types (https://elixir.bootlin.com/linux/latest/A/ident/designator_prio). The current set of udev rules in sg3_utils (https://github.com/hreinecke/sg3_utils/blob/master/scripts/55-scsi-sg3_id.rules) don't use the kernel's wwid attribute; they parse VPD 83 and 80 instead and prioritize types 36, 35, 32, and 2 over type 8. udev's "scsi_id" tool, historically the first attempt to implement a priority for this, doesn't look at the SCSI name attribute at all: https://github.com/systemd/systemd/blob/main/src/udev/scsi_id/scsi_serial.c There's a "fallback" logic in multipath-tools in case udev doesn't provide a WWID: https://github.com/opensvc/multipath-tools/blob/a41a61e8482def33e3ca8c9e3639ad2c37611551/libmultipath/discovery.c#L1040 -- Dr. Martin Wilck <mwilck@xxxxxxxx>, Tel. +49 (0)911 74053 2107 SUSE Software Solutions Germany GmbH HRB 36809, AG Nürnberg GF: Felix Imendörffer -- dm-devel mailing list dm-devel@xxxxxxxxxx https://listman.redhat.com/mailman/listinfo/dm-devel
