Re: [PATCH v3 3/4] btrfs: introduce new read_policy device

[Josef Bacik <josef@toxicpanda.com>]

On 1/11/21 4:41 AM, Anand Jain wrote:
> Read-policy type 'device' and device flag 'read-preferred':
> 
> The read-policy type device picks the device(s) flagged as
> read-preferred for reading stripes 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 advanced 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).
> 
> Note:
> 
> If the read-policy type is set to 'device', but there isn't any device
> which is flagged as read-preferred, then stripe 0 is used for reading.
> 
> The device replacement won't migrate the read-preferred flag to the new
> replace the target device.
> 
> As of now, this is an in-memory only feature.
> 
> It's pointless to set the read-preferred flag on the missing device, as
> IOs aren't submitted to the missing device.
> 
> If there is more than one read-preferred device in a chunk, the read IO
> shall go to the stripe 0 as of now.
> 
> Usage example:
> 
> Consider a typical two disks raid1.
> 
> Configure devid1 for reading.
> 
> $ echo 1 > devinfo/1/read_preferred
> $ cat devinfo/1/read_preferred
> 1
> $ cat devinfo/2/read_preferred
> 0
> 
> $ pwd
> /sys/fs/btrfs/12345678-1234-1234-1234-123456789abc
> 
> $ cat read_policy
> [pid] device
> $ echo device > ./read_policy
> $ cat read_policy
> pid [device]
> 
> Now read IOs are sent to devid 1 (sdb).
> 
> $ echo 3 > /proc/sys/vm/drop_caches
> $ md5sum /btrfs/YkZI
> 
> $ iostat -zy 1 | egrep 'sdb|sdc' (from another terminal)
> sdb              50.00     40048.00         0.00      40048          0
> 
> Change the read-preferred device from devid 1 to devid 2 (sdc).
> 
> $ echo 0 > ./devinfo/1/read_preferred
> 
> [ 3343.918658] BTRFS info (device sdb): reset read preferred on devid 1 (1334)
> 
> $ echo 1 > ./devinfo/2/read_preferred
> 
> [ 3343.919876] BTRFS info (device sdb): set read preferred on devid 2 (1334)
> 
> $ echo 3 > /proc/sys/vm/drop_caches
> $ md5sum /btrfs/YkZI
> 
> Further read ios are sent to devid 2 (sdc).
> 
> $ iostat -zy 1 | egrep 'sdb|sdc' (from another terminal)
> sdc              49.00     40048.00         0.00      40048          0
> 
> Signed-off-by: Anand Jain <anand.jain@oracle.com>

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef