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Date:   Wed, 7 Jun 2023 10:07:00 -0700
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Subject: Re: [PATCH] scsi: sd: support specify probe type of build-in driver
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To:     Jianlin Lv <iecedge@gmail.com>
Cc:     jejb@linux.ibm.com, martin.petersen@oracle.com, paulmck@kernel.org,
        bp@suse.de, peterz@infradead.org, will@kernel.org,
        rdunlap@infradead.org, kim.phillips@amd.com, rostedt@goodmis.org,
        wyes.karny@amd.com, jianlv@ebay.com, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-scsi@vger.kernel.org
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From:   Bart Van Assche <bvanassche@acm.org>
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On 6/7/23 08:55, Jianlin Lv wrote:
> 1. MegaRAID adapters associated with 24 local disks. The disks are named
> sequentially as "sda," "sdb," and so on, up to "sdx."
> 2. STAT controllers associated with the root disk, named "sdy."
> 
> Both the MegaRAID adapters and the SATA controller (PCH) are accessed via
> the PCIe bus. In theory, depending on their PCIe bus ID in ascending order,
> the devices should be initialized in ascending order as well.

Hmm ... I don't think there is anything that prevents the PCIe maintainer
from changing the PCIe probing behavior from synchronous to asynchronous?
In other words, I don't think it is safe to assume that PCIe devices are
always scanned in the same order.

> For cloud deployment, the local volume provisioner detects and creates PVs
> for each local disk (from sda to sdx) on the host, and it cleans up the
> disks when they are released.
> This requires the logical names of the disks to be deterministic.

I see two possible solutions:
- Change the volume provisioner such that it uses disk references that do
   not depend on the probing order, e.g. /dev/disk/by-id/...
- Implement an algorithm in systemd that makes disk names predictable.
   An explanation of how predictable names work for network interfaces is
   available here: https://wiki.debian.org/NetworkInterfaceNames. The
   systemd documentation about predictable network names is available here:
   https://www.freedesktop.org/software/systemd/man/systemd.net-naming-scheme.html

These alternatives have the advantage that disk scanning remains asynchronous.

Thanks,

Bart.