Re: Unreliable disk detection order in 5.x

[Simon Kirby <sim@hostway.ca>]

On Sun, Nov 07, 2021 at 11:51:45AM -0800, Bart Van Assche wrote:

> On 11/6/21 19:24, Simon Kirby wrote:
> > This occurs regardless of the CONFIG_SCSI_SCAN_ASYNC setting, and
> > also with scsi_mod.scan=sync on vendor kernels. All of these disks
> > are coming from the same driver and card.
> > 
> > I understand that using UUIDs, by-id, etc., is an option to work
> > around this, but then we would have to push IDs for disks in every
> > server to our configuration management. It does not seem that this
> > change is really intentional.
> 
> SCSI disk detection is asynchronous on purpose since a long time. The most
> recent commit I know of that changed SCSI disk scanning
> behavior is commit f049cf1a7b67 ("scsi: sd: Rely on the driver core for
> asynchronous probing").
> 
> Please use one of the /dev/disk/by-*/* identifiers as Damien requested.

Hi Bart,

So, we're using DRBD on top of these, which means by-uuid is not
available; we can use only by-id and by-path. by-id is dependent on disk
models and serial numbers, and by-path is dependent on PCI bus details.
Both are going to be a good deal more work to maintain, since they're
both not just a simple enumeration.

I did try 5.14.17 with f049cf1a7b67 (and a065c0faacb1) reverted, and it
does indeed restore the behaviour where sd* order appears to be reliable.
Scan time (time until systemd starts) is within 4ms across 3 boots with
and without the revert, but this is just our particular case.

I don't fully understand the scan process here, but I can understand the
challenges in trying to parallelize it and still end up with a consistent
enumerated list.

I guess you would agree that removing sd* entirely would not be an option
because they've existed forever historically, but at the same time, the
only time they really "work" now are as symlink targets for by-*, and in
the case where only one disk exists at boot time. Do I have this right?

Simon-