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From:   Matteo Croce <mcroce@linux.microsoft.com>
Date:   Thu, 25 Mar 2021 18:29:10 +0100
Message-ID: <CAFnufp3zKa+9K-hsV5vRkv-w8y-1nZioq_bFAnzaxs9RoP+sDA@mail.gmail.com>
Subject: Re: [PATCH -next 1/5] block: add disk sequence number
To:     Matthew Wilcox <willy@infradead.org>
Cc:     linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Lennart Poettering <lennart@poettering.net>,
        Luca Boccassi <bluca@debian.org>, Jens Axboe <axboe@kernel.dk>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Damien Le Moal <damien.lemoal@wdc.com>,
        Tejun Heo <tj@kernel.org>,
        =?UTF-8?Q?Javier_Gonz=C3=A1lez?= <javier@javigon.com>,
        Niklas Cassel <niklas.cassel@wdc.com>,
        Johannes Thumshirn <johannes.thumshirn@wdc.com>,
        Hannes Reinecke <hare@suse.de>
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On Mon, Mar 15, 2021 at 10:05 PM Matthew Wilcox <willy@infradead.org> 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@microsoft.com>
> > >
> > > 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.

Regards,
-- 
per aspera ad upstream