From: Bart Van Assche <firstname.lastname@example.org> To: Randy Dunlap <email@example.com>, Bradley LaBoon <firstname.lastname@example.org>, email@example.com, linux-scsi <firstname.lastname@example.org> Subject: Re: SCSI device probing non-deterministic in 5.3 Date: Fri, 4 Oct 2019 08:39:01 -0700 Message-ID: <email@example.com> (raw) In-Reply-To: <firstname.lastname@example.org> 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.
next prev parent reply index Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-10-03 20:32 Bradley LaBoon 2019-10-03 21:19 ` Randy Dunlap 2019-10-04 15:39 ` Bart Van Assche [this message] 2019-10-08 20:20 ` Bradley LaBoon
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