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

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Message-ID: <f6d6622d-a9a0-d7de-b5af-b7a885ee1b61@infradead.org>
Date:   Thu, 3 Oct 2019 14:19:37 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     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

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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?
> 
> I am not subscribed to the list, so please CC me on any replies.
> 
> Thank you!
> Bradley LaBoon
> 


-- 
~Randy