FS#63984 - [linux] 5.3 kernel not loading scsi devices in bus order
Attached to Project:
Arch Linux
Opened by L. Bradley LaBoon (lb.laboon) - Tuesday, 01 October 2019, 23:22 GMT
Last edited by freswa (frederik) - Friday, 21 February 2020, 21:54 GMT
Opened by L. Bradley LaBoon (lb.laboon) - Tuesday, 01 October 2019, 23:22 GMT
Last edited by freswa (frederik) - Friday, 21 February 2020, 21:54 GMT
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Details
Description:
Starting with kernel 5.3, SCSI devices are no longer loaded/processed in bus order, and instead appear to be processed randomly. In my case, I have an Archlinux QEMU guest with two block SCSI devices (one ext4 main device, and one swap device) and I explicitly set the bus ordering via QEMU command line arguments so that within the guest I can refer to them by their device names (sda, sdb, etc). Prior to 5.3, the device names would always be related to their bus order. For example, if my SCSI bus IDs are 0:0:0:0 and 0:0:1:2, then 0:0:0:0 would always be sda and 0:0:1:2 would always be sdb. Starting with 5.3, this is no longer the case and the result is different with each reboot. ~50% of the time the guest boots into an emergency shell due to the devices being in the unexpected order. I have attached a snippet of the kernel boot log during an unsuccessful boot, where it can be seen that device ID 0:0:1:2 gets processed/named before 0:0:0:0. 
boot.log
(1.9 KiB)
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This task depends upon
Closed by freswa (frederik)
Friday, 21 February 2020, 21:54 GMT
Reason for closing: None
Additional comments about closing: This seems pretty stalled to me. If it's still an issue, please fill a re-open request. Thank you :)
Friday, 21 February 2020, 21:54 GMT
Reason for closing: None
Additional comments about closing: This seems pretty stalled to me. If it's still an issue, please fill a re-open request. Thank you :)
[1] Is recommended to avoid exactly issues such as this.
[1] https://wiki.archlinux.org/index.php/Persistent_block_device_naming
Unfortunately, using UUID (et al) as identifiers isn't an option in our case since multiple VMs are deployed from a common base image, and we don't know what the partition UUIDs are going to be at the time the image is made. This is why we explicitly order devices on the bus so we can rely on the device names being consistent.
I just built a kernel with those two commits reverted and it seems to be working perfectly.
So it sounds like the new asynchronous device probing is what's biting us, and from the way the commits are worded it sounds like that's the intended functionality.
I'll see if I can get a thread started in the LKML. This change really hurts those of us who deploy VMs from base images, because as I mentioned above we can't bake UUIDs into the image because we don't know what they're going to be ahead of time.
https://lore.kernel.org/lkml/CAHk-=wiP4K8DRJWsCo=20hn_6054xBamGKF2kPgUzpB5aMaofA%40mail.gmail.com/