Re: [PATCH 15/17] btrfs: calculate file system wide queue limit for zoned mode

[Christoph Hellwig <hch@lst.de>]

On Fri, Sep 02, 2022 at 10:56:40AM +0900, Damien Le Moal wrote:
> > -	/* Max size to emit ZONE_APPEND write command */
> > +	/* Constraints for ZONE_APPEND commands: */
> > +	struct queue_limits limits;
> >  	u64 max_zone_append_size;
> 
> Can't we get rid of this one and have the code directly use
> fs_info->limits.max_zone_append_sectors through a little helper doing a
> conversion to bytes (a 9 bit shift) ?

Well, the helper would be a little more complicated, doing three
different shifts, a max3 and and ALIGN_DOWN.  That's why I thought
I'd rather cache the value then recalculating it on every write.  But
either way would be entirely feasible.

> This does:
> 
> 	t->max_zone_append_sectors = min(t->max_zone_append_sectors,
>                                         b->max_zone_append_sectors);
> 
> So if we are mixing zoned and non-zoned devices in a multi-dev volume,
> we'll end up with max_zone_append_sectors being 0. The previous code
> prevented that.
> 
> Note that I am not sure if it is allowed to mix zoned and non-zoned drives
> in the same volume. Given that we have a fake zone emulation for non-zoned
> drives with zoned btrfs, I do not see why it would not work. But I may be
> wrong.

Yes, this could be problematic.  I wonder if we need to initialize
max_zone_append_sectors to max_hw_sectors by default and use a separate
flag if it is actually supported in the block layer if we want to
support that.