Re: Discontiguous folios/pagesets

[Darrick J. Wong]

On Sat, Aug 28, 2021 at 01:27:29PM -0600, Andreas Dilger wrote:
> On Aug 28, 2021, at 1:04 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > 
> > The current folio work is focused on permitting the VM to use
> > physically contiguous chunks of memory.  Both Darrick and Johannes
> > have pointed out the advantages of supporting logically-contiguous,
> > physically-discontiguous chunks of memory.  Johannes wants to be able to
> > use order-0 allocations to allocate larger folios, getting the benefit
> > of managing the memory in larger chunks without requiring the memory
> > allocator to be able to find contiguous chunks.  Darrick wants to support
> > non-power-of-two block sizes.
> 
> What is the use case for non-power-of-two block sizes?  The main question
> is whether that use case is important enough to add the complexity and
> overhead in order to support it?

For copy-on-write to a XFS realtime volume where the allocation extent
size (we support bigalloc too! :P) is not a power of two (e.g. you set
up a 4 disk raid5 with 64k stripes, now the extent size is 192k).

Granted, I don't think folios handling 192k chunks is absolutely
*required* for folios; the only hard requirement is that if any page in
a 192k extent becomes dirty, the rest have to get written out all the
same time, and the cow remap can only happen after the last page
finishes writeback.

--D

> Cheers, Andreas
> 
> 
> 
> 
>