Hi, Anand Jain
Cc: Louis Jencka

> Read-policy type 'device' and device flag 'read-preferred':
> 
> The read-policy type device picks the device(s) flagged as
> read-preferred for reading chunks of type raid1, raid10,
> raid1c3 and raid1c4.
> 
> A system might contain SSD, nvme, iscsi or san lun, and which are all
> a non-rotational device, so it is not a good idea to set the read-preferred
> automatically. Instead device read-policy along with the read-preferred
> flag provides an ability to do it manually. This advance tuning is
> useful in more than one situation, for example,
>  - In heterogeneous-disk volume, it provides an ability to manually choose
>     the low latency disks for reading.
>  - Useful for more accurate testing.
>  - Avoid known problematic device from reading the chunk until it is
>    replaced (by marking the other good devices as read-preferred).

It is still OK to auto for the most common case of the mixed of ssd and
hdd?

I am trying 'manually if failed to auto' with a 'u8' var rather than a 'bool'
var.

There are 2 patch I am working but yet not completed.

and someone of them is based on 'btrfs: balance RAID1/RAID10 mirror
selection' from Louis Jencka louis@waffle.tech

Feel free to merge them into your patch as a new one please.

Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2020/10/27