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

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Date:   Mon, 15 Feb 2021 14:40:26 +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 Sat, Feb 13, 2021 at 12:36:02AM +0100, Michał Mirosław wrote:
> 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.

I like the idea of storing numbers and simply checking them. I use it in
v2 - though probably in a different form, and I will most likely move
the whole logic around checking device types to separate functions, to
not bloat btrfs_open_one_device() and the others too much.