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.j...@oracle.com>
Reviewed-by: Josef Bacik <jo...@toxicpanda.com> Thanks, Josef
