Re: [RFC PATCH 2/2] mm: introduce budgt control in readahead

[Zhaoyang Huang <huangzhaoyang@gmail.com>]

On Fri, May 10, 2024 at 11:18 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Fri, May 10, 2024 at 10:43:20AM +0800, Zhaoyang Huang wrote:
> > Thanks for the prompt. I did some basic research on soft RAID and
> > wonder if applying the bps limit on /dev/md0 like below could make
> > this work.
>
> No.  Look at btrfs' raid support, for example.  it doesn't use md0.
If I understand the below command correctly, btrfs uses one of the
volumes within RAID as the mount block device, not /dev/md0. However,
I think this is a problem of blkio.throttle rather than this commit
which means this readahead budget control will work accordingly as
long as blkio.throttle's parameter is configured correctly(eg. 50/50
on sdb and sdc)

mkfs.btrfs -m raid0 -d raid0 /dev/sdb /dev/sdc
mount -t btrfs /dev/sdb /mnt/btr



>
> > I didn't find information about 'RAID internally'. Could we set the
> > limit on the root device(the one used for mount) to manage the whole
> > partition without caring about where the bio finally goes? Or ask the
> > user to decide if to use by making sure the device they apply will not
> > do RAID?
>
> No.