On 2020-02-19 11:29, Anand Jain wrote:
A new read policy 'device' is introduced with this patch, which when
set
can pick only the device flagged as read_preferred for reading. This
tunable is for the advance users and the testers, which can make sure
that
reads are read from the device they prefer for chunks of type raid1,
raid10, raid1c3 and raid1c4.
The default read policy is pid which can be changed to device as below.
$ pwd
/sys/fs/btrfs/12345678-1234-1234-1234-123456789abc
$ cat read_policy; echo device > ./read_policy; cat read_policy
[pid] device
pid [device]
One or more devices which are favored for reading should set the flag
read-preferred. In an example below a typical two disk raid1, devid1 is
configured as read preferred.
$ echo 1 > devinfo/1/read_preferred
$ cat devinfo/1/read_preferred; cat devinfo/2/read_preffered
Typo: should be read_preferred
1
0
So now when the file is read, the read IO would prefer device(s) with
read_preferred flags for reading.
$ echo 3 > /proc/sys/vm/drop_caches; md5sum /btrfs/YkZI
Since the devid 1 (sdb) is our read preferred device, the reads are set
to sdb only.
$ iostat -zy 1 | egrep 'sdb|sdc' (from another terminal)
sdb 50.00 40048.00 0.00 40048 0
$ echo 0 > ./devinfo/1/read_preferred; echo 1 >
./devinfo/2/read_preferred;
[ 3343.918658] BTRFS info (device sdb): reset read preferred on devid 1
(1334)
[ 3343.919876] BTRFS info (device sdb): set read preferred on devid 2
(1334)
$ echo 3 > /proc/sys/vm/drop_caches; md5sum /btrfs/YkZI
Since now we changed the read preferred from devid 1 (sdb) to 2 (sdc),
now all the read IO goes to sdc.
$ iostat -zy 1 | egrep 'sdb|sdc' (from another terminal)
sdc 49.00 40048.00 0.00 40048 0
Whenever there isn't any read preferred device(s) or if more than one
stripe is marked as read preferred device then this read policy shall
use the stripe 0 for reading.
Should we consider the situation where more than one device is preferred
(perhaps for a future patch) - e.g. devid1 is HDD, devid2 is SSD, devid3
is SSD and data is RAID1C3?
Will there be a warning when this fallback to stripe 0 happens? Although
I imagine that would either always display on mount before
read_preferred is set or flood dmesg for every read.
Perhaps fallback to the %pid policy to give some form of balancing would
be a better default?
--
Steven Davies
