[dm-devel] question about block-throttle on data device of dm-thin pool

[Hou Tao]

Hi, all.

I am trying to test block-throttle on dm-thin devices. I find the throttling
on dm-thin device is OK, but the throttling doesn't work for the data device
of dm-thin pool.

The following is my test case:
#!/bin/sh

dmsetup create pool --table '0 41943040 thin-pool /dev/vdb /dev/vda \
	128 6553 1 skip_block_zeroing
dmsetup message /dev/mapper/pool 0 'create_thin 1'
dmsetup create thin_1 --table '0 41943040 thin /dev/mapper/pool 1'

mp=/thin_1
mkfs.xfs /dev/mapper/thin_1
mount /dev/mapper/thin_1 $mp

cg=/sys/fs/cgroup/blkio/test
mkdir -p $cg
# get the block device id of the data device
data_dev=$(dmsetup table /dev/mapper/pool | awk '{print $5}')
echo "${data_dev} 1048576" > $cg/blkio.throttle.write_bps_device
echo $$ > $cg/cgroup.procs
dd if=/dev/zero of=$mp/zero bs=1M count=1 oflag=direct

I read the dm-thin code roughly and find out that most bios are submitted
by the workqueue of thin pool instead of the dd process which initiates the
O_DIRECT write operations. The bios belong to the block cgroup "blkcg_root"
instead of the created block cgroup "test" in test case, so the write
limitation of blkcg "test" doesn't work.

In order to make the throttling work out, can we save the original block
cgroup info of the deferred bios and use the saved block cgroup info to
submit the bios ? Is the method reasonable and is there a better way to
complete the throttling on the data device of the thin pool ?

Thanks.

Tao