[dm-devel] max_sectors_kb limitations with VDO and dm-thin

[Zdenek Kabelac]

Dne 19. 04. 19 v 16:40 Ryan Norwood napsal(a):
> 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?

Hi

If you require highest performance - I'd suggest to avoid using VDO.
VDO replaces performance with better space utilization.
It works on 4KiB block - so by design it's going to be slow.

I'd also probably not mix 2 provisioning technologies together - there
is nontrivial amount of problematic states when the whole device stack
runs out of real physical space.

Regards

Zdenek