Re: Discontiguous folios/pagesets

[Andreas Dilger]

> On Aug 30, 2021, at 12:28 PM, Darrick J. Wong <djwong@xxxxxxxxxx> wrote:
> 
> 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.

OK, they are still multiples of PAGE_SIZE.  That wasn't clear, I thought
these were byte-granular IOs or something...

Cheers, Andreas

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