Re: max_sectors_kb limitations with VDO and dm-thin

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Apr 19 2019 at 10:40am -0400,
Ryan Norwood <ryan.p.norwood@gmail.com> wrote:

>    We have been using dm-thin layered above VDO and have noticed that our
>    performance is not optimal for large sequential writes as max_sectors_kb
>    and max_hw_sectors_kb for all thin devices are set to 4k due to the VDO
>    layer beneath.
>    This effectively eliminates the performance optimizations for sequential
>    writes to skip both zeroing and COW overhead when a write fully overlaps a
>    thin chunk as all bios are split into 4k which always be less than the 64k
>    thin chunk minimum.
>    Is this known behavior? Is there any way around this issue?

Are you creating the thin-pool to use a 4K thinp blocksize?  If not,
I'll have to look to see why the block core's block limits stacking
would impose these limits of the underlying data device.

>    We are using RHEL 7.5 with kernel 3.10.0-862.20.2.el7.x86_64.

OK, please let me know what the thin-pool's blocksize is.

Thanks,
Mike