Canvassing for network filesystem write size vs page size

[Matthew Wilcox]

On Thu, Aug 05, 2021 at 05:35:33PM +0100, David Howells wrote:
> With Willy's upcoming folio changes, from a filesystem point of view, we're
> going to be looking at folios instead of pages, where:
> 
>  - a folio is a contiguous collection of pages;
> 
>  - each page in the folio might be standard PAGE_SIZE page (4K or 64K, say) or
>    a huge pages (say 2M each);

This is not a great way to explain folios.

If you're familiar with compound pages, a folio is a new type for
either a base page or the head page of a compound page; nothing more
and nothing less.

If you're not familiar with compound pages, a folio contains 2^n
contiguous pages.  They are treated as a single unit.

>  - a folio has one dirty flag and one writeback flag that applies to all
>    constituent pages;
> 
>  - a complete folio currently is limited to PMD_SIZE or order 8, but could
>    theoretically go up to about 2GiB before various integer fields have to be
>    modified (not to mention the memory allocator).

Filesystems should not make an assumption about this ... I suspect
the optimum page size scales with I/O bandwidth; taking PCI bandwidth
as a reasonable proxy, it's doubled five times in twenty years.

> Willy is arguing that network filesystems should, except in certain very
> special situations (eg. O_SYNC), only write whole folios (limited to EOF).

I did also say that the write could be limited by, eg, a byte-range
lease on the file.  If the client doesn't have permission to write
a byte range, then it doesn't need to write it back.