linux-kernel - Re: [PATCH] drivers: base: Introduce a new kernel parameter driver_sync

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Message-ID: <2023120814-failing-trio-7c8b@gregkh>
Date:   Fri, 8 Dec 2023 08:15:26 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     jejb@...ux.ibm.com, martin.petersen@...cle.com, rafael@...nel.org,
        linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: [PATCH] drivers: base: Introduce a new kernel parameter
 driver_sync_probe=

On Fri, Dec 08, 2023 at 02:49:39PM +0800, Yafang Shao wrote:
> On Fri, Dec 8, 2023 at 1:36 PM Greg KH <gregkh@...uxfoundation.org> wrote:
> >
> > On Thu, Dec 07, 2023 at 08:36:56PM +0800, Yafang Shao wrote:
> > > On Thu, Dec 7, 2023 at 8:12 PM Greg KH <gregkh@...uxfoundation.org> wrote:
> > > >
> > > > On Thu, Dec 07, 2023 at 07:59:03PM +0800, Yafang Shao wrote:
> > > > > On Thu, Dec 7, 2023 at 6:19 PM Greg KH <gregkh@...uxfoundation.org> wrote:
> > > > > >
> > > > > > On Wed, Dec 06, 2023 at 10:08:40PM +0800, Yafang Shao wrote:
> > > > > > > On Wed, Dec 6, 2023 at 9:31 PM Greg KH <gregkh@...uxfoundation.org> wrote:
> > > > > > > >
> > > > > > > > On Wed, Dec 06, 2023 at 11:53:55AM +0000, Yafang Shao wrote:
> > > > > > > > > After upgrading our kernel from version 4.19 to 6.1, certain regressions
> > > > > > > > > occurred due to the driver's asynchronous probe behavior. Specifically,
> > > > > > > > > the SCSI driver transitioned to an asynchronous probe by default, resulting
> > > > > > > > > in a non-fixed root disk behavior. In the prior 4.19 kernel, the root disk
> > > > > > > > > was consistently identified as /dev/sda. However, with kernel 6.1, the root
> > > > > > > > > disk can be any of /dev/sdX, leading to issues for applications reliant on
> > > > > > > > > /dev/sda, notably impacting monitoring systems monitoring the root disk.
> > > > > > > >
> > > > > > > > Device names are never guaranteed to be stable, ALWAYS use a persistant
> > > > > > > > names like a filesystem label or other ways.  Look at /dev/disk/ for the
> > > > > > > > needed ways to do this properly.
> > > > > > >
> > > > > > > The root disk is typically identified as /dev/sda or /dev/vda, right?
> > > > > >
> > > > > > Depends on your system.  It can also be identified, in the proper way,
> > > > > > as /dev/disk/by-uuid/eef0abc1-4039-4c3f-a123-81fc99999993 if you want
> > > > > > (note, fake uuid, use your own disk uuid please.)
> > > > > >
> > > > > > Why not do that?  That's the most stable and recommended way of doing
> > > > > > things.
> > > > >
> > > > > Adapting to this change isn't straightforward, especially for a large
> > > > > fleet of servers. Our monitoring system needs to accommodate and
> > > > > adjust accordingly.
> > > >
> > > > Agreed, that can be rough.  But as this is an issue that was caused by a
> > > > scsi core change, perhaps the scsi developers can describe why it's ok.
> > > >
> > > > But really, device naming has ALWAYS been known to not be
> > > > deterministic, which is why Pat and I did all the driver core work 20+
> > > > years ago so that you have the ability to properly name your devices in
> > > > a way that is deterministic.  Using the kernel name like sda is NOT
> > > > using that functionality, so while it has been nice to see that it has
> > > > been stable for you for a while, you are playing with fire here and will
> > > > get burned one day when the firmware in your devices decide to change
> > > > response times.
> > >
> > > I agree that using UUID is a better approach. However, it's worth
> > > noting that the widely used IO monitoring tool 'iostat' faces
> > > challenges when working with UUIDs. This indicates that there's a
> > > significant amount of work ahead of us in this aspect.
> >
> > That indicates that iostat needs to be fixed as this has been an option
> > that people rely on for 20+ years now.  Or use a better tool :)
> 
> The issue arises when a disk contains multiple partitions, such as
> /dev/sda1 and /dev/sda2. In this case, using 'iostat -j UUID' can only
> display 'sda' since only its partitions possess UUIDs. Uncertain how
> to address it yet.

Then use one of the other many other unique ids that are in /dev/disk/
today.  You have loads of things to choose from:
	$ ls /dev/disk/
	by-diskseq  by-id  by-label  by-partlabel  by-partuuid  by-path  by-uuid

You have a plethera of choices here, use whatever works best for your
systems.  This is a userspace decision to make, not a kernel one, as
this is a policy choice of yours.

good luck!

greg k-h