Re: blkio weights not honoured with lvm setup and on amazon ebs

[nborisov <n.borisov.lkml-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 25.04.2016 20:53, Himadri Sarkar wrote:
> Hi,
> 
> I was doing some experiments to test out blkio with the following
> setups and was unable to get the expected behavior. It will be great
> if someone can help me understand what might have gone wrong with my
> setup.
> 
> Setup 1
> hardware: d2.xlarge machine on aws (It has 3 * 2 TB hdd)
> logical volume: setup an lvm to use 2 disks out of 3
> file system: xfs
> IO Scheduler: cfq
> blkio weights: test1 has weight 1000, test2 has weight 500
> 
> Now when I run the tests as given in
> https://www.kernel.org/doc/Documentation/cgroup-v1/blkio-controller.txt
> using dd (I only executed read tests on already written files)
> I found that both the processes were getting equal time share
> (blkio.time) and serviced bytes (blkio.io_service_bytes) irrespective
> of differential weights.
> 
> Now when I modified the setup by not having lvm setup and just
> creating a 2TB file system out of a single hdd it worked i.e. serviced
> bytes were in the ratio 2 : 1
> 
> On the other hand when I tested read with fio
> 
> specifically
> 
> fio --randrepeat=1 --ioengine=libaio --direct=1 --gtod_reduce=1
> --name=test2 --filename=file2 --bs=4k --iodepth=64 --size=4G
> --readwrite=randrw --rwmixread=100
> 
> Then also it worked even with the lvm setup. Which makes be believe
> that with Setup 1 it is not working due to buffered reads. (But then
> the same thing also worked without lvm)

This is a well-known limitation to LVM, which supposedly was fixed in a
recent kernel. For more information check this email thread:
https://www.redhat.com/archives/dm-devel/2016-February/msg00183.html

Then a patch was proposed in the following thread:
https://www.redhat.com/archives/dm-devel/2016-March/msg00006.html

This was tested by me and the proportional-based limits were working,
provided that the devices hosting the LVM VG were using CFQ as their io
scheduler.

Regards,
Nikolay