linux-kernel - Re: SCSI device probing non-deterministic in 5.3

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Date:   Fri, 4 Oct 2019 08:39:01 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Randy Dunlap <rdunlap@...radead.org>,
        Bradley LaBoon <blaboon@...ode.com>,
        linux-kernel@...r.kernel.org,
        linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: SCSI device probing non-deterministic in 5.3

On 10/3/19 2:19 PM, Randy Dunlap wrote:
> [add linux-scsi mailing list]
> 
> On 10/3/19 1:32 PM, Bradley LaBoon wrote:
>> Hello, LKML!
>>
>> Beginning with kernel 5.3 the order in which SCSI devices are probed and
>> named has become non-deterministic. This is a result of a patch that was
>> submitted to add asynchronous device probing (specifically, commit
>> f049cf1a7b6737c75884247c3f6383ef104d255a). Previously, devices would
>> always be probed in the order in which they exist on the bus, resulting
>> in the first device being named 'sda', the second device 'sdb', and so on.
>>
>> This is important in the case of mass VM deployments where many VMs are
>> created from a single base image. Partition UUIDs cannot be used in the
>> fstab of such an image because the UUIDs will be different for each VM
>> and are not known in advance. Normally you can't rely on device names
>> being consistent between boots, but with QEMU you can set the bus order
>> of each block device and thus we currently use that to control the
>> device order in the guest. With the introduction of the aforementioned
>> patch this is no longer possible and the device ordering is different on
>> every boot, resulting in the guest booting into an emergency shell
>> unless the devices randomly happen to be loaded in the expected order.
>>
>> I have created a patch which reverts back to the previous behavior, but
>> I wanted to open this topic to discussion before posting it. I'm not
>> totally familiar with the low-level details of SCSI device probing, so I
>> don't know if the non-deterministic device order was the intended
>> behavior of the patch or just a side-effect. If that is the intended
>> behavior then is there perhaps some other way to ensure a consistent
>> device ordering for a guest VM?

Have you already had a look at the /dev/disk/by-path directory? An 
example of the contents of that directory:

$ (cd /dev/disk/by-path && ls -l | grep /s)
lrwxrwxrwx 1 root root  9 Oct  3 16:49 pci-0000:00:02.0-ata-1 -> ../../sda
lrwxrwxrwx 1 root root  9 Oct  3 16:49 pci-0000:00:08.0-scsi-0:0:0:1 -> 
../../sr0

Have you considered to use these soft links in /etc/fstab?

In case using these links would be impractical: have you considered to 
add a udev rule that creates H:C:I:L soft links under a subdirectory of 
/dev, that makes these soft links point at the /dev/sd* device nodes and 
to use these soft links in /etc/fstab? That's probably a much more 
elegant solution than what has been proposed above.

As one can see the information that is needed to implement such a udev 
rule is already available in sysfs:

$ (cd /sys/class/scsi_device && ls -ld */device/block/*)
drwxr-xr-x 9 root root 0 Oct  3 16:48 2:0:0:1/device/block/sr0
drwxr-xr-x 9 root root 0 Oct  3 16:48 3:0:0:0/device/block/sda

Bart.