[linux-lvm] how to set higher then 128 nr_requests on LV's

[Mike Snitzer]

On Sun, May 13 2018 at  8:47pm -0400,
Pankaj Agarwal <pankaj at releasemanager.in> wrote:

>    Hi,
>    How do i set the nr_request value for LV's as it's not writable like other
>    drives on a linux system.
>    LV's are set as dm-0 and dm-1 on my system.
>     #cat /sys/block/dm-0/queue/nr_requests
>    128
>    # echo 256 > /sys/block/dm-0/queue/nr_requests
>    -bash: echo: write error: Invalid argument

In the block layer, bio-based devices cannot have their nr_requests
changed, see: block/blk-sysfs.c:queue_requests_store()

So any bio-based DM device (e.g. linear target) won't be allowed to
change nr_requests.

The only way to change this would be from the bottom up:
1) change the underlying request-based device(s) (e.g. /dev/sda)
2) dmsetup suspend + dmsetup resume any bio-based DM device(s) that are
   stacked ontop of the request-based device(s) you changed in 1)
   -- this will restack the queue_limits from the bottom up; so the DM
   device will then reflect the underlying devices' limits.

Mike