[dm-devel] max_sectors_kb limitations with VDO and dm-thin
Mike Snitzer
snitzer at redhat.com
Wed Apr 24 19:45:30 UTC 2019
On Fri, Apr 19 2019 at 10:40am -0400,
Ryan Norwood <ryan.p.norwood at 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
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