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

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Message-ID: <20210212233602.GA30441@qmqm.qmqm.pl>
Date:   Sat, 13 Feb 2021 00:36:02 +0100
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Michal Rostecki <mrostecki@...e.de>
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 Fri, Feb 12, 2021 at 06:26:41PM +0000, Michal Rostecki wrote:
> 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.
[...]
> Inverting the order of those `if` checks would break the other
> permuitations which start with rotational disks.

But not if you would add:

if (adding first device)
	fs_devices->rotating = rotating;

before the checks.

But them, there is a simpler way: count how many rotating vs non-rotating
devices there are while adding them. Like:

rotating ? ++n_rotating : ++n_fixed;

And then on remove you'd have it covered.

Best Regards
Michał Mirosław