Re: [PATCH] btrfs: zoned: limit ordered extent to zoned append size

[Johannes Thumshirn <Johannes.Thumshirn@wdc.com>]

On 24/05/2021 01:05, Damien Le Moal wrote: 
>>> +	if (fs_info && fs_info->max_zone_append_size)
>>> +		max_bytes = ALIGN_DOWN(fs_info->max_zone_append_size,
>>> +				       PAGE_SIZE);
>>
>> Why is the alignment needed? Are the max zone append values expected to
>> be so random? Also it's using memory-related value for something that's
>> more hw related, or at least extent size (which ends up on disk).

I did the ALIGN_DOWN() call because we want to have complete pages added.

> It is similar to max_hw_sectors: the hardware decides what the value is. So we
> cannot assume anything about what max_zone_append_size is.
> 
> I think that Johannes patch here limits the extent size to the HW value to avoid
> having to split the extent later one. That is efficient but indeed is a bit of a
> layering violation here.

Damien just brought up a good idea: what about a function to lookup the max extent
size depending on the block group. For regular btrfs it'll for now just return 
BTRFS_MAX_EXTENT_SIZE, for zoned btrfs it'll return 
ALIGN_DOWN(fs_info->max_zone_append_size, PAGE_SIZE) and it also gives us some 
headroom for future improvements in this area.