linux-kernel - Re: [PATCH RFC 4/6] btrfs: Check if the filesystem is has mixed type of devices

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Date:   Fri, 12 Feb 2021 18:26:41 +0000
From:   Michal Rostecki <mrostecki@...e.de>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        "open list:BTRFS FILE SYSTEM" <linux-btrfs@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Michal Rostecki <mrostecki@...e.com>
Subject: Re: [PATCH RFC 4/6] btrfs: Check if the filesystem is has mixed type
 of devices

On Wed, Feb 10, 2021 at 05:08:05AM +0100, Michał Mirosław wrote:
> On Tue, Feb 09, 2021 at 09:30:38PM +0100, Michal Rostecki wrote:
> > From: Michal Rostecki <mrostecki@...e.com>
> > 
> > Add the btrfs_check_mixed() function which checks if the filesystem has
> > the mixed type of devices (non-rotational and rotational). This
> > information is going to be used in roundrobin raid1 read policy.a
> [...]
> > @@ -669,8 +699,12 @@ static int btrfs_open_one_device(struct btrfs_fs_devices *fs_devices,
> >  	}
> >  
> >  	q = bdev_get_queue(bdev);
> > -	if (!blk_queue_nonrot(q))
> > +	rotating = !blk_queue_nonrot(q);
> > +	device->rotating = rotating;
> > +	if (rotating)
> >  		fs_devices->rotating = true;
> > +	if (!fs_devices->mixed)
> > +		fs_devices->mixed = btrfs_check_mixed(fs_devices, rotating);
> [...]
> 
> Since this is adding to a set, a faster way is:
> 
> if (fs_devices->rotating != rotating)
> 	fs_devices->mixed = true;
> 
> The scan might be necessary on device removal, though.
> 

Actually, that's not going to work in case of appenging a rotational
device when all previous devices are non-rotational.

  if (rotating)
        fs_devices->rotating = true;
  if (fs_devices->rotating != rotating)
        fs_devices->mixed = true;

If all devices are non-rotational, we start with the following
attributes:

fs_devices->rotating: false
fs_devices->mixed: false

Then, while appending a rotational disk, we have:

  rotating = true;
  if (rotating)                         // if (true)
        fs_devices->rotating = true;    // overriding with `true`
  if (fs_devices->rotating != rotating) // if (true != true), which is false
        fs_devices->mixed = true;       // NOT EXECUTED

So we end up fs_devices->mixed being `false`, despite having a mixed
array.

Inverting the order of those `if` checks would break the other
permuitations which start with rotational disks.

Therefore, to cover all cases, I think we need a full check, always.

Regards,
Michal