Re: [PATCH 3/3] btrfs: zoned: limit active zones to max_open_zones

[Damien Le Moal <dlemoal@kernel.org>]

On 7/7/25 11:44 AM, Naohiro Aota wrote:
> When there is no active zone limit, we can technically write into any
> number of zones at the same time. However, exceeding the max open zones can
> degrade performance. To prevent this, set the max_active_zones to
> bdev_max_open_zones() if there is no active zone limit.
> 
> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
> ---
>  fs/btrfs/zoned.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
> index 9c354e84ab07..bdcfabcb35e7 100644
> --- a/fs/btrfs/zoned.c
> +++ b/fs/btrfs/zoned.c
> @@ -418,6 +418,8 @@ int btrfs_get_dev_zone_info(struct btrfs_device *device, bool populate_cache)
>  		zone_info->nr_zones++;
>  
>  	max_active_zones = bdev_max_active_zones(bdev);
> +	if (!max_active_zones)
> +		max_active_zones = bdev_max_open_zones(bdev);

	max_active_zones = min_not_zero(bdev_max_active_zones(bdev),
					bdev_max_open_zones(bdev));

And what if the device has no limits at all ? (e.g. no max active limit and no
max open limit). In this case, max_active_zones will be zero. Should we perhaps
set in that case a default max_active zones ?

For information, the block layer zone write plugging defaults to create a 128
zones pull of write plugs if the device has no limits
(BLK_ZONE_WPLUG_DEFAULT_POOL_SIZE in block/blk-zoned.c). I would recommend
using that limit here too. So something like this:

/* Default number of max active zones when the device has no limits. */
#define BTRFS_ZONE_DEFAULT_MAX_ACTIVE	128

	max_active_zones = min_not_zero(bdev_max_active_zones(bdev),
					bdev_max_open_zones(bdev));
	if (!max_active_zones &&
	    bdev_nr_zones(bdev) > BTRFS_ZONE_DEFAULT_MAX_ACTIVE)
		max_active_zones = BTRFS_ZONE_DEFAULT_MAX_ACTIVE;


>  	if (max_active_zones && max_active_zones < BTRFS_MIN_ACTIVE_ZONES) {
>  		btrfs_err(fs_info,
>  "zoned: %s: max active zones %u is too small, need at least %u active zones",


-- 
Damien Le Moal
Western Digital Research