On 3/25/21 6:29 PM, Matteo Croce wrote:

On Mon, Mar 15, 2021 at 10:05 PM Matthew Wilcox [off-list ref] wrote:

quoted

On Mon, Mar 15, 2021 at 08:18:24PM +0000, Matthew Wilcox wrote:

quoted

On Mon, Mar 15, 2021 at 09:02:38PM +0100, Matteo Croce wrote:

quoted

From: Matteo Croce <redacted>

Add a sequence number to the disk devices. This number is put in the
uevent so userspace can correlate events when a driver reuses a device,
like the loop one.

Should this be documented as monotonically increasing?  I think this
is actually a media identifier.  Consider (if you will) a floppy disc.
Back when such things were common, it was possible with personal computers
of the era to have multiple floppy discs "in play" and be prompted to
insert them as needed.  So shouldn't it be possible to support something
similar here -- you're really removing the media from the loop device.
With a monotonically increasing number, you're always destroying the
media when you remove it, but in principle, it should be possible to
reinsert the same media and have the same media identifier number.

So ... a lot of devices have UUIDs or similar.  eg:

$ cat /sys/block/nvme0n1/uuid
e8238fa6-bf53-0001-001b-448b49cec94f

https://linux.die.net/man/8/scsi_id (for scsi)

Hi,

I don't have uuid anywhere:

matteo@saturno:~$ ll /dev/sd?
brw-rw---- 1 root disk 8,  0 feb 16 13:24 /dev/sda
brw-rw---- 1 root disk 8, 16 feb 16 13:24 /dev/sdb
brw-rw---- 1 root disk 8, 32 feb 16 13:24 /dev/sdc
brw-rw---- 1 root disk 8, 48 feb 16 13:24 /dev/sdd
brw-rw---- 1 root disk 8, 64 mar  4 06:26 /dev/sde
brw-rw---- 1 root disk 8, 80 feb 16 13:24 /dev/sdf
matteo@saturno:~$ ll /sys/block/*/uuid
ls: cannot access '/sys/block/*/uuid': No such file or directory

mcroce@t490s:~$ ll /dev/nvme0n1
brw-rw----. 1 root disk 259, 0 25 mar 14.22 /dev/nvme0n1
mcroce@t490s:~$ ll /sys/block/*/uuid
ls: cannot access '/sys/block/*/uuid': No such file or directory

I find it only on a mdraid array:

$ cat /sys/devices/virtual/block/md127/md/uuid
26117338-4f54-f14e-b5d4-93feb7fe825d

I'm using a vanilla 5.11 kernel.

The 'uuid' is optional for NVMe devices, and indeed not even present for 
other device types.
Use the 'wwid' attribute, which contains a unique identifier for all 
nvme devices:

# cat /sys/block/nvme*/wwid
nvme.8086-4356504436343735303034323430304e474e-564f303430304b45464a42-00000001
nvme.8086-4356504436343735303034363430304e474e-564f303430304b45464a42-00000001
uuid.3c6500ee-a775-4c89-b223-e9551f5a9f7a

and for SCSI the wwid is part of the SCSI device:
# cat /sys/block/sd*/device/wwid
naa.600508b1001ce2e648a35b6ec14a3996

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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