Re: [PATCH -next 1/5] block: add disk sequence number

[Hannes Reinecke]

On 3/25/21 6:29 PM, Matteo Croce wrote:
> On Mon, Mar 15, 2021 at 10:05 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > On Mon, Mar 15, 2021 at 08:18:24PM +0000, Matthew Wilcox wrote:
> > > On Mon, Mar 15, 2021 at 09:02:38PM +0100, Matteo Croce wrote:
> > > > From: Matteo Croce <mcroce@xxxxxxxxxxxxx>
> > > >
> > > > 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@xxxxxxx                              +49 911 74053 688
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