[dm-devel] dm-thin: Why is DATA_DEV_BLOCK_SIZE_MIN_SECTORS set to 64k?

[Zdenek Kabelac]

Dne 18.5.2018 v 01:36 Eric Wheeler napsal(a):
> Hello all,
> 
> Is there a technical reason that DATA_DEV_BLOCK_SIZE_MIN_SECTORS is
> limited to 64k?
> 
> I realize that the metadata limits the maximum mappable pool size, so it
> needs to be bigger for big pools---but it is also the minimum COW size.
> 
> Looking at the code this is enforced in pool_ctr() but isn't used anywhere
> else in the code.  Is it strictly necessary to enforce this minimum?
> 


Hi

Selection of 64k  was chosen as compromise between used space for metadada, 
locking contention, kernel memory usage and overall speed performance.

If there is a case, where using 4K chunks for snapshot is giving a major 
advantage, there is still availability to use old snapshot target.


Regards

Zdenek