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

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Date:   Wed, 10 Feb 2021 05:08:05 +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 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.

> -	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);

Duplication. Maybe pull all this into a function?

Best Regards,
Michał Mirosław