Re: [PATCH RFC 6/6] btrfs: Add roundrobin raid1 read policy

[Michał Mirosław]

On Tue, Feb 09, 2021 at 09:30:40PM +0100, Michal Rostecki wrote:
[...]
> For the array with 3 HDDs, not adding any penalty resulted in 409MiB/s
> (429MB/s) performance. Adding the penalty value 1 resulted in a
> performance drop to 404MiB/s (424MB/s). Increasing the value towards 10
> was making the performance even worse.
> 
> For the array with 2 HDDs and 1 SSD, adding penalty value 1 to
> rotational disks resulted in the best performance - 541MiB/s (567MB/s).
> Not adding any value and increasing the value was making the performance
> worse.
> 
> Adding penalty value to non-rotational disks was always decreasing the
> performance, which motivated setting it as 0 by default. For the purpose
> of testing, it's still configurable.
[...]
> +	bdev = map->stripes[mirror_index].dev->bdev;
> +	inflight = mirror_load(fs_info, map, mirror_index, stripe_offset,
> +			       stripe_nr);
> +	queue_depth = blk_queue_depth(bdev->bd_disk->queue);
> +
> +	return inflight < queue_depth;
[...]
> +	last_mirror = this_cpu_read(*fs_info->last_mirror);
[...]
> +	for (i = last_mirror; i < first + num_stripes; i++) {
> +		if (mirror_queue_not_filled(fs_info, map, i, stripe_offset,
> +					    stripe_nr)) {
> +			preferred_mirror = i;
> +			goto out;
> +		}
> +	}
> +
> +	for (i = first; i < last_mirror; i++) {
> +		if (mirror_queue_not_filled(fs_info, map, i, stripe_offset,
> +					    stripe_nr)) {
> +			preferred_mirror = i;
> +			goto out;
> +		}
> +	}
> +
> +	preferred_mirror = last_mirror;
> +
> +out:
> +	this_cpu_write(*fs_info->last_mirror, preferred_mirror);

This looks like it effectively decreases queue depth for non-last
device. After all devices are filled to queue_depth-penalty, only
a single mirror will be selected for next reads (until a read on
some other one completes).

Have you tried testing with much more jobs / non-sequential accesses?

Best Reagrds,
Michał Mirosław